Were the pixels black/white and more noticeable on plane textures? I had an issue where I would hear my speakers pop (I hope) then the screen would get like you described and Framerates would die (until I looked away from plane). It persisted onto the desktop until I reset the computer. I think it had to do with too much Overclocking in my system, but I'm not positive, especially if you don't OC. I just had to replace my GPU due to it randomly dieing (odd since it was EVGA and under 1.5 years old) but it had been several months since the incident had happened.
Not real sure what it is, but if you OC try turning it down, and it's possible (although I have no real evidence) the GPU is considering dieing.