could be the 76 is normal and you had a better firmware that pushed 81 th but stole it with a bad pool switch.
Yup it's pretty normal for infected firmware to overclock your miner to make the most money out of it, they know it's only a matter of time before the owner finds out and solves the problem, however, there is also a possibility that the other firmware version did better tunning on the individual chips and was able to achieve better hashrate.
OP, you say the miner was working fine, so we know it didn't come infected, you really have to figure out where the hell it came from, you sure have a virus/malware sitting somewhere in your local network, it scans for miners and brute force itself in, let me guess, your password was the default or was easy to guess?
