My computer goes wonky and does this after running AH H2H host for a few hours. I also start getting graphic glitches, a blue flashing stairstepped pattern at the bottom left edge of the screen.
Possible fix for you: HT told me to make sure the monitor is defined properly under display properties. If you select a generic 1600x1200 supervga monitor (or whatever generic monitor type matches your monitor's capabilities), it should work, otherwise find and select the exact monitor type you have.
For me, a reboot fixes the problem so it's either a system stability problem or the game making my system forget what kind of monitor I have. I don't know which it is and since I'm using third-party tweaked drivers, I don't know if HT should waste time chasing my problem down. Selecting the right monitor might work for you if it isn't already done.
Oh yea... The solution to many graphics or sound problems is often to re-install directX, so go to
http://www.microsoft.com/directx and download/install the latest version.