It may be your video card resetting itself. It's known as VPU Recover on Ati cards; I'm not sure what it's called on Nvidia.
Pretty much what it does, is when it detects that the video card has stopped responding, it resets it. This will make your monitor shut off, and usually come back on. I've never had it just stop like that.
What video card do you use?
Imagination is more important than knowledge.