[Coda]

It does sound like it might be an overheating issue. It's safe to run a computer with the case open, so go ahead and open it up and you can watch the fans to see if they're running when they should be.

If the fan isn't spinning, or if it's spinning slowly, then the fan probably needs to be replaced. If the fan is running at a proper speed, it may be that there's damage to the card itself (potentially heat damage) and it becomes unstable when it warms up, even if it's not technically overheating.