Brief update. According to the reports I received so far, the problem seems to occur with several 3D cards and several drivers, but only under Windows XP. So it's apparently not directly related to the driver or 3D hardware.

I had it on one 8600 GT system here also, but that turned out to be temperature dependent, and thus was obviously a hardware problem. I can not observe it here on any other 8600 or 8800 system.

The pattern of the problem suggests that nVidia can probably do nothing on their end. My guess is now also that it's a DirectX bug. The next step for me would be to test if a change of the DirectX version has an effect on this problem.

Has anyone ever observed this on a non-XP OS?