My stats went down from 230GH/s to 170GH/s and have remained at that level for 36 hours now, whereas the other half of my hash power going to slush has remained constant, as has the S3 itself which is sitting at 465GH/s.
Thinking back, I had a similar issue with both Slush and BTCG when I first fired up my S3+s. Initially I let the pools set their own difficulty levels, but when I changed it manually to a minimum of 256 for each worker on each pool I started getting better reported hash rates. Each S3+ is balanced between the two pools, and each is set up as a seperate worker. Of course, the optimum setting may be different for an S3 and so YMMV, but it could be worth experimenting.