I had the same thing until I added more power or used better PSUs, even going from using a piss-poor 850.  If you have other PUSs handy, it might be worth it to test that and rule it out to be sure.  Other things could be network problems, pool problems, using the same miner, sub-optimal miner difficulty settings, or even the already-noted S3 mining rate instability.  I'm sure you've already ruled out most of those, just throwing them out there anyway...