Like I said before, It's likely a driver issues. This is indicated by the computer running faster in non sli. Try DDU and reinstall drivers. Also if it's possible, do a fresh windows install. I can't gaurentee a fix but I know that ddu saved my butt from some glitches yesterday. If that doesn't work I'd contact your manufacturer. Idk how much they will actually know... in fact I doubt that they will even know what sli is so.. maby contact nvidia. Idk if they have a support line but it's your best bet. Good luck!
edit: also try to re seat the cards and see what fps you get on the other one in non sli. Make sure they both play with similar fps in non sli.