Ah - when I've seen that happen, it's been caused by the graphics driver crashing and then recovering. There's usually a notification in the lower right to that effect, though. You might want to have a look in windows Event Viewer (in the Application and System sections) to see if there's an error at the same time. My guess at this point is driver crash, though. Why it'd crash is another question - the possibilities range from "bug in the driver" to "current install of the driver is corrupted" to "the hardware is on the fritz". If it's the second, then a clean reinstall of the graphics drivers might help, though that always carries a touch of risk.