When does this happen while playing? After a few minutes/hours?
Seems like its heat related though. Its one of the graphics pipes overheating within the core, it looks like. The type of artifacts you start to get when overclocking too much in ATI Tool.
Check the temperature within the Nividia graphic drivers. You'll probably have to download Nvidia NTune, which has the temperature monitor built in.
When you download it, and find it, set it to save the temperature, and play the game. When it starts happening, minimize and look at the graph. If it reaches extreme highs, you know thats the problem.
Anything higher than 120 degrees C is a problem. Anything lower, a GPU can cope with, unless you've overclocked it?