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?
Some units do and some don't, try a lower clock such a 212.5 as others have identified. I have one that runs steady at 218.5 and another that does not like the stock clock but runs fine at 212.5, and is reliable at that speed.