Back to the issue at hand, is the GeForce 4 one of the cards with an onboard fan? If so, the bearings in those things have been known to go out once in a while so you may check that it's still working.
It's either a heat issue or something fundamentally wrong with an internal component like the onboard memory. IC's are pretty robust though, so unless the product has a history of problems I'd almost have to go with overheating. Normally the first thing I'd ask is are you overclocking, but I'm sure you know better than that if it's goofing up. I'd seriously doubt that there's something wrong with any part of the card or mobo that's actually exposed.