Yeah, that is your graphics card artifacting. It is more than likely overheating, so you might want to take it out and give the heat-sink a thorough cleaning. If you've got it OCd, then back it down. You can run a few stress tests to see how well it handling temps in real time with some free benchmarking software (I use FurMark). If you're running a beta driver, that could also be an issue on the way that the card is handling itself, so you might consider rolling back to the last stable release.