I hope the card itself has a fan on it? If not, that's your first problem.
If it does, it may not be seated properly, or something (that's really hard to do with a graphics card cooler though).
But ya, it sounds like a simple overheating problem to me. My Gainward would overheat occasionally, but usually only after a couple hours of NWN or Morrowind. They seem to use substandard coolers. Which is too bad because I really liked the card.