Does anyone have the weird problem where normal clocked s3's give 440 GH/s quite well, and overclocked S3's (to 250), give...440 Gh/s?

They do seem to start out faster, with a higher rate initially, but always seem to go back down to this level. I just don't understand why they would do that. Hardware errors are low in all cases. I'm using P2Pool, but I don't think that should have anything to do with it. Does anyone shed some light on the issue?