It sounds like the card is overheating. Understandable, given the card in question and what your doing on it. UT/Quake and the like are intense hardware-stressing games that "beat on" lower-end cards. I originally had the same card in a rig I built for my GF. Just playing Mooshu Tiles at 1024x768, the card would get too hot to touch. Returned the sucker. She's now using my old Voodoo3000 PCI.
Reinstall the drivers and see if that helps any. If it doesn't, it's time for a new card. GF MX400 is affordable and will greatly outperform the card you have now.