I have been getting pretty poor performance in many games I own lately. Games like Crysis 2 with DX11 and the high res texture pack I used to get like 80-90 FPS but now I get maybe 30-50. Need for Speed Shift 2 I get around 28 FPS where my friend with significantly lower hardware gets well above that. The Witcher 2 (ubersampling off) I get like 35 FPS and I know I should be getting much higher than that.
So what your saying is that even though you've made no alterations to your PC, performance has suddenly dropped?
You might want to check your OC has reset - My 4.6 OC on my 2500k did about 2 weeks back and I immediately noticed a performance hit.
Seeing as youre running an SLI system, and it would seem like your performance, using Crysis as example, has almost halved, Id be checking to see if one of your 6950's has died. Try each on them on their own and see what goes on.