I've experienced what sounds like the same issue with a few different video cards while watching fullscreen video with XV acceleration. In my case at least, it seems to be related to the issue described here:
With the video card in powermizer performance mode 1 or 2, I don't seem to experience the artifacts. On performance mode 0 however, where the GPU and memory clocks are lowest, I experience XV artifacts (red checkerboard pattern flashes briefly) and I have occasional GPU lockups on the desktop. The symptoms of a GPU lockup are the entire X interface freezing except the mouse cursor, and the "Recovered GPU Errors" figure in the nvidia control panel increments by one.
I wonder if your issue is identical to my own...
- If you playback video with non-XV accelleration, e.g. OpenGL, does the issue occur then?
- Are you seeing any accompanying GPU lockups? E.g. if you X session has been running for a long time, do you have any soft GPU crashes ("Recovered GPU Errors")?
- Do you see any strange messages in the output of dmesg related to "NVIDIA"?