In almost every instance, so far, that I have seen this, it was related to the sound card/sound card drivers and/or processes running in the background.
Looking at the process list in the task manager does not cut it. Processes can fire off and die very quickly and not show in the task manager at all.
But, you can simply go to the desktop, shut down all programs, bring up the task manager and watch the CPU usage. With your CPU, you should never see any CPU usage show up.
If you do, then something is running without your knowledge. Probably spyware. Also note, if you are using some type of screen saver/wall paper utility that randomly rotates the background, it will cause the same problem.
In the game, how much texture/video memory is being used versus what is available?
Shoot me a cpoy of your DXDIAG and I will be glad to take a look and see if anything sticks out that could be the culprit.
