I've been getting this problem lately in the middle of GWJ Team Fortress 2 sessions. The game runs fine for an hour or two and then occasionally the game will freeze up. The screen goes black and the in game audio and voice chat will repeat the same one second phrase for a while.
I can recover from the error by hitting Ctrl-Alt-Delete and killing the game from Task Manager. The game runs fine for a while and sometimes the problem happens again and sometimes it doesn't. My hunch is that something might be overheating, but does someone have a more informed opinion?