have you tried to update the driver?
also, the card itself might be overheating. if there's a fan on it, check to make sure the fan is actually working when the computer is turned on. overheating can cause a whole list of problems. if the fan seems to be working, and the driver is updated, and it's still doing it, there is likely something wrong with the card itself.
if your mobo has onboard graphics, disable the video card, and use the onboard graphics (or a different card) to see if the problem is still occurring.
hmm...
also, check in msconfig to make sure the application that controls the card is running, and working properly.
good luck. let me know how you make out.