I think it's the ATI drivers in general. I run into this issue at least once a month when playing borderlands. At that time, my video freezes for abot 20 seconds, then goes blank for a second or two, then resumes just fine. Later when I check the event log it says the display driver stopped working and has been restarted.
I'm not sure why yours doesn't restart on it own though. It may have something to do with your x-fire setup.
Alright I'll be sure to play with that a bit thanks!