can someone shed some light on one of my S3s?

The chips are all ok, no errors reported (as in "x" or "-") and what happens, when I power off the S3 it starts hashing at 450gh/s but veeery slowly over 18-24 hours drops down to 405gh/s average (thats using stats from the miner itself). Why is this happening ? Powering it off and restarting is bit of a pain as the only way to do it is to pull the cables manually out in order not to shut down other miners.