I'm also having the issue with the S3 hashrate dropping off.  It generally starts around 435Gh but drops to around 390-400GHs.  I'm at work now and I'm looking at my S3 on Slush's pool and I'm currently showing -

Total average hash rate in last 60 minutes: 381070 Mhash/s 

Has anyone found a solution to this or had word from BitMain?