I've had something like that happen to me while playing another game. Turned out my fan fell off my videocard and it was overheating. The thing literally melted off the spindle and dangled there... it happened to my geforce 4400 and more recently to my 6600gt (which I did overclock a little). Either way, it is definitely related to your graphic card, and I would make sure firstly, it's fan is still on (lol) and/or that the computer is getting enough airflow through it.