The first thing I do when I encounter any problem with my computer is think back to the last thing I changed before the issue popped up. Literally the last thing you changed. I don`t mean empty the trash can. I mean I installed a game, or grabbed some driver `update` and installed it or changed the slot a card was in, or whatever. That usually gives me a hint of what is going wrong.
A few tips though, you can double check you have the most up-to-date drivers, try changing the card slot, ummm.... not much else I can think of other than getting another card and plugging it in to check the hardware. I know that is something no one wants to hear, but it`s a possibility.

Good Luck