whole screen goes blank

I was able to play for about 1 hour, then my whole screen went blank. I get no error, and no reaction when i hit any buttons. I hear a click like when a graphic card makes a major resolution change. It seems to do it a lot more now, sometimes 5 min, sometimes it takes 20 min then it goes blank and i have to reboot.

Otherwise playing just fine, all looks good and runs smooth except for this intermittent problem.

anyone else see this?

I'm running at 1024 x 768
Geforce Nvidia- latest drivers (october)
Updated directx
1 gig of memory