Pools don't receive a report of miner hashrate, they get difficulty 1 share submissions and must extrapolate what the miner hashrate may be.
Miner share finding is as capricious as block finding on the Bitcoin network. It has even more variance if the variable difficulty algorithm is setting your miner to a higher difficulty than 1 given your relatively low hashrate. Pools generally display hashrate over a small period of time, such as the last 10 minutes - the rate is better used just to see that miners are working.
You should analyze for yourself if the number of shares submitted over a period of time truly decreases, or if not decreasing, if the pool is properly crediting you for all submitted shares:
cgminer --sharelog sharelog.txt
The first entry in a log line is the epoch time in seconds. Inspect the number of shares submitted in a 10000 second period after miner startup vs a 10000 second period when the pool displays a lower rate than expected. Repeat for a different time interval.
If hashrate indeed decreases, many things might be going on, from OS entering a low power state, USB driver problems, overheating/bad hashing in the ASIC, etc.
