Everything sounds just fine. Are your graphics drivers up to date? That's the most common problem I tend to see from people that tend to have the problem you described. Go here:
http://www.nvidia.com/Download/index.aspx?lang=en-us
Also, you may want to download EVGA's tuning tool, Precision X.
http://www.evga.com/precision/
See what temps and clock speed you're getting from your two cards in SLI, and get back to me.