I don't know for sure, but that looks to be an issue with your card itself. Maybe it is over heating or something. Have you tried running other graphically intensive games for long periods of time to see if it happens in them as well?
First think I would try there would be to upgrade your video drivers and reboot your PC. You might also want to check and make sure your PC doesn't have a bunch of dust build-up in it causing overheating. If so, get some compressed air and clean that bastard out
