When the problem showed up? If it's a new card I would go to annoy the dealer
If the card used to work right, I would check what changed on the computer since last time it worked fine. To see if it overheats, I would keep an eye on the temperature of the card and stress it with other things, maybe some benchmarking tool or so. And see what happens. Also, I try to check the internet to see if anyone reported a simular problem with your card and the mainboard. (somethimes just a time google helps) And like Mo said, it often helps to start from a clean partition and rebuild the system.
About the manufacturer,
I think ati is doing it a bit better at the moment than nvidia if you run windows. If you always use windows, try ati. Still, there's almost no difference between them. (and I never had problems with my nvidia cards) If you don't run windows,
never buy ati cards. The drivers for non-windows (lets say for xfree86 and xorg) are just incomplete and bad (I still want to blow up my ati-mobile chip in my laptop, it keeps me from running a load of things). Nivdia is fine, the driver isn't opensource, but it works at least.
-Mel
