I have updated to 1.6.6 on this machine, but I also ran the log to see if we could tell anything about the hash rate drop. This is what it shows in the log (doesn't seem to show anything to me):
[2018-08-10 04:32:13] json_receive: {"id":1,"jsonrpc":"2.0","result":{"status":"OK"},"error":null}
[2018-08-10 04:32:13] miner_result: Pool accepted result 0x000014DB
[2018-08-10 04:32:14] hashrate: GPU0: 2083 H/s [BUS:7]
[2018-08-10 04:32:14] hashrate: GPU1: 2083 H/s [BUS:21]
[2018-08-10 04:32:14] hashrate: GPU2: 2086 H/s [BUS:10]
[2018-08-10 04:32:14] hashrate: GPU3: 2086 H/s [BUS:3]
[2018-08-10 04:32:14] hashrate: GPU4: 2088 H/s [BUS:18]
[2018-08-10 04:32:14] hashrate: GPU5: 2087 H/s [BUS:13]
[2018-08-10 04:32:14] hashrate: Total: 12513 H/s
[2018-08-10 04:32:34] json_send: {"id":1,"jsonrpc": "2.0","method":"login","params":{"login":"41mmdqPwrQEDHxPhK7MJVJ2PcHSPLeGn7M5gQWom2CfjfXdhcWhHqUCCn2BEDfCyyxPp6qf52u77pHcxRPMpiYezGrFKKS7.vega6/
[email protected]","pass":"
[email protected]","agent":"SRBMiner Cryptonight AMD GPU miner/1.6.5","rigid":""}}
[2018-08-10 04:32:34] json_receive: {"id":1,"jsonrpc":"2.0","result":{"id":"1","job":{"blob":"0707afb7b5db0519b80d4e46e22ef0c36fef4e2e9574d3ab0d4f6602f98a3e5baa0e8a52255f6e0000000034062f153f1f8d4cee2fde160a6d22fdcf891ba4ad5a807e4eccc5b86f08054515","job_id":"12156","target":"cf8b0000"},"status":"OK"},"error":null}
[2018-08-10 04:32:34] pool_have_job: Pool sent a new job (ID: 12156)
[2018-08-10 04:32:37] miner_result: Sending user result to pool
[2018-08-10 04:32:37] json_send: {"id":1,"jsonrpc": "2.0","method":"submit","params":{"id":"1","job_id":"12156","nonce":"16595595","result":"1ff6ca63413b4864ba931d8d4b5710aaee5940fb0e6c2b44c6112b680c450000"}}
[2018-08-10 04:32:37] json_receive: {"id":1,"jsonrpc":"2.0","result":{"status":"OK"},"error":null}
[2018-08-10 04:32:37] miner_result: Pool accepted result 0x0000450C
[2018-08-10 04:32:44] miner_result: Sending user result to pool
[2018-08-10 04:32:44] json_send: {"id":1,"jsonrpc": "2.0","method":"submit","params":{"id":"1","job_id":"12156","nonce":"82765555","result":"d328d60322c7d861cdf620db6ce4ee08df5de992df7f5657c89b3526413b0000"}}
[2018-08-10 04:32:44] json_receive: {"id":1,"jsonrpc":"2.0","result":{"status":"OK"},"error":null}
[2018-08-10 04:32:44] miner_result: Pool accepted result 0x00003B41
[2018-08-10 04:32:48] miner_result: Sending user result to pool
[2018-08-10 04:32:48] json_send: {"id":1,"jsonrpc": "2.0","method":"submit","params":{"id":"1","job_id":"12156","nonce":"8adcaa2a","result":"8b3b0703fea0f008b58247da8c915f31071720d4b128dc2416c1adfaf86a0000"}}
[2018-08-10 04:32:49] json_receive: {"id":1,"jsonrpc":"2.0","result":{"status":"OK"},"error":null}
[2018-08-10 04:32:49] miner_result: Pool accepted result 0x00006AF8
[2018-08-10 04:32:58] miner_result: Sending user result to pool
[2018-08-10 04:32:58] json_send: {"id":1,"jsonrpc": "2.0","method":"submit","params":{"id":"1","job_id":"12156","nonce":"d602abea","result":"c94a8b47fecbf432f38e69df8f1cd4c1716bcb00485f6e19ef0246b275800000"}}
[2018-08-10 04:32:58] json_receive: {"id":1,"jsonrpc":"2.0","result":{"status":"OK"},"error":null}
[2018-08-10 04:32:58] miner_result: Pool accepted result 0x00008075
[2018-08-10 04:33:00] miner_result: Sending user result to pool
[2018-08-10 04:33:00] json_send: {"id":1,"jsonrpc": "2.0","method":"submit","params":{"id":"1","job_id":"12156","nonce":"86af5555","result":"11315681ce885e602428495bb18f3b4ba7a2d0df48aa3c859d7dcf8974030000"}}
[2018-08-10 04:33:00] json_receive: {"id":1,"jsonrpc":"2.0","result":{"status":"OK"},"error":null}
[2018-08-10 04:33:00] miner_result: Pool accepted result 0x00000374
[2018-08-10 04:33:02] miner_result: Sending user result to pool
[2018-08-10 04:33:02] json_send: {"id":1,"jsonrpc": "2.0","method":"submit","params":{"id":"1","job_id":"12156","nonce":"90660000","result":"d908aabaea94c89d5dd512aeb4eca85c34feb27f323e200b17865d0c73520000"}}
[2018-08-10 04:33:03] json_receive: {"id":1,"jsonrpc":"2.0","result":{"status":"OK"},"error":null}
[2018-08-10 04:33:03] miner_result: Pool accepted result 0x00005273
[2018-08-10 04:33:08] miner_result: Sending user result to pool
[2018-08-10 04:33:08] json_send: {"id":1,"jsonrpc": "2.0","method":"submit","params":{"id":"1","job_id":"12156","nonce":"fe24abaa","result":"ea4a1ef8d95c52105767111cbb0e216866ddc2654db7b7ddf1a7792d6b790000"}}
[2018-08-10 04:33:08] json_receive: {"id":1,"jsonrpc":"2.0","result":{"status":"OK"},"error":null}
[2018-08-10 04:33:08] miner_result: Pool accepted result 0x0000796B
[2018-08-10 04:33:16] miner_result: Sending user result to pool
[2018-08-10 04:33:16] json_send: {"id":1,"jsonrpc": "2.0","method":"submit","params":{"id":"1","job_id":"12156","nonce":"8d9b00c0","result":"491be38f77445e0a9413451f146a9c5394622eb7859ee013fe07f2f7f02b0000"}}
[2018-08-10 04:33:16] json_receive: {"id":1,"jsonrpc":"2.0","result":{"status":"OK"},"error":null}
[2018-08-10 04:33:16] miner_result: Pool accepted result 0x00002BF0
[2018-08-10 04:33:31] json_receive: {"jsonrpc":"2.0","method":"job","params":{"blob":"0707ecb7b5db0519b80d4e46e22ef0c36fef4e2e9574d3ab0d4f6602f98a3e5baa0e8a52255f6e000000009cee94de92c6bfe3a071f77c6653b8f811a6440e4781cb46d4bccf53657cda1b15","job_id":"12157","target":"cf8b0000"}}
[2018-08-10 04:33:31] pool_have_job: Pool sent a new job (ID: 12157)
[2018-08-10 04:33:36] miner_result: Sending user result to pool
[2018-08-10 04:33:36] json_send: {"id":1,"jsonrpc": "2.0","method":"submit","params":{"id":"1","job_id":"12157","nonce":"1a5f5515","result":"ff1c2d60455b35d1ff2a4740cc1f72b88e71b9a3637edb69c26b1b1441710000"}}
[2018-08-10 04:33:36] json_receive: {"id":1,"jsonrpc":"2.0","result":{"status":"OK"},"error":null}
[2018-08-10 04:33:36] miner_result: Pool accepted result 0x00007141
[2018-08-10 04:33:38] miner_result: Sending user result to pool
[2018-08-10 04:33:38] json_send: {"id":1,"jsonrpc": "2.0","method":"submit","params":{"id":"1","job_id":"12157","nonce":"92665515","result":"75bd8a801b2f8596fa3458d3063414cacd5ea13d8b014a9bb1a8468a7f6f0000"}}
[2018-08-10 04:33:38] json_receive: {"id":1,"jsonrpc":"2.0","result":{"status":"OK"},"error":null}
[2018-08-10 04:33:38] miner_result: Pool accepted result 0x00006F7F
[2018-08-10 04:33:46] miner_result: Sending user result to pool
[2018-08-10 04:33:46] json_send: {"id":1,"jsonrpc": "2.0","method":"submit","params":{"id":"1","job_id":"12157","nonce":"61320000","result":"f28a4bade094eab3aa76bb7a98ac1c8898f34d3e93667670b2c0184dc9460000"}}
[2018-08-10 04:33:46] json_receive: {"id":1,"jsonrpc":"2.0","result":{"status":"OK"},"error":null}
[2018-08-10 04:33:46] miner_result: Pool accepted result 0x000046C9
[2018-08-10 04:34:03] miner_result: Sending user result to pool
[2018-08-10 04:34:03] json_send: {"id":1,"jsonrpc": "2.0","method":"submit","params":{"id":"1","job_id":"12157","nonce":"34720040","result":"c20df53f861b1f3e5673904365e58d1cadadb8f247b0b7d64c4c9f16c1640000"}}
[2018-08-10 04:34:04] json_receive: {"id":1,"jsonrpc":"2.0","result":{"status":"OK"},"error":null}
[2018-08-10 04:34:04] miner_result: Pool accepted result 0x000064C1
[2018-08-10 04:34:05] miner_result: Sending user result to pool
[2018-08-10 04:34:05] json_send: {"id":1,"jsonrpc": "2.0","method":"submit","params":{"id":"1","job_id":"12157","nonce":"9223ab2a","result":"0f5ef63281ddfe09d1d74cd9768dc85ee6ebe9c814a4b18634a8339b3b690000"}}
[2018-08-10 04:34:06] json_receive: {"id":1,"jsonrpc":"2.0","result":{"status":"OK"},"error":null}
[2018-08-10 04:34:06] miner_result: Pool accepted result 0x0000693B
[2018-08-10 04:34:09] miner_result: Sending user result to pool
[2018-08-10 04:34:09] json_send: {"id":1,"jsonrpc": "2.0","method":"submit","params":{"id":"1","job_id":"12157","nonce":"ba32abaa","result":"bc37c38fd1af11dc11698f67d1b0690c4d1de30c5513b871ce58f6fcbc1b0000"}}
[2018-08-10 04:34:09] json_receive: {"id":1,"jsonrpc":"2.0","result":{"status":"OK"},"error":null}
[2018-08-10 04:34:09] miner_result: Pool accepted result 0x00001BBC
[2018-08-10 04:34:13] miner_result: Sending user result to pool
[2018-08-10 04:34:13] json_send: {"id":1,"jsonrpc": "2.0","method":"submit","params":{"id":"1","job_id":"12157","nonce":"8045abaa","result":"26b80269a1b37ab7c263054eede646d2d8609a2c71c0c2ce6485f4ccef4c0000"}}
[2018-08-10 04:34:13] json_receive: {"id":1,"jsonrpc":"2.0","result":{"status":"OK"},"error":null}
[2018-08-10 04:34:13] miner_result: Pool accepted result 0x00004CEF
[2018-08-10 04:34:17] hashrate: GPU0: 1973 H/s [BUS:7]
[2018-08-10 04:34:17] hashrate: GPU1: 1978 H/s [BUS:21]
[2018-08-10 04:34:17] hashrate: GPU2: 1977 H/s [BUS:10]
[2018-08-10 04:34:17] hashrate: GPU3: 1963 H/s [BUS:3]
[2018-08-10 04:34:17] hashrate: GPU4: 1972 H/s [BUS:18]
[2018-08-10 04:34:17] hashrate: GPU5: 1979 H/s [BUS:13]
[2018-08-10 04:34:17] hashrate: Total: 11842 H/s
[2018-08-10 04:34:31] json_receive: {"jsonrpc":"2.0","method":"job","params":{"blob":"0707a8b8b5db0519b80d4e46e22ef0c36fef4e2e9574d3ab0d4f6602f98a3e5baa0e8a52255f6e00000000bc59d150965596546db1e074d22f030f65f650e2d3ed8ce59da414a45ad9309515","job_id":"12158","target":"cf8b0000"}}