First thing I would do is try it with a different monitor (or change the refresh rate of your current monitor to see if it acts differently). It sounds like your card is over-heating, but its hard to say.
If your computer starts off fine and then progressively gets worse after videos/gaming, then I would definitely say your video card is overheating. If this is the case, try leaving the side of your case open and aim a fan at your computer to try to bring down the temp.
I had a monitor once that got hot at certain resolutions and would start flipping out on me. But that was a CRT, so who knows.
-Aug