The screensaver was topping out the CPU. The CPU would overheat and then the machine would power down to save the CPU. The fix was EXACTLY as you see at Cnet's video at http://reviews.cnet.com/4520-11319_7-6240575-1.html
I am running Windows XP. When the screen saver mode goes on after the PC has been idle for a while I notice that sometimes when you hit the spacebar either nothing happens at all or the PC shuts down completely and I have to turn it back on again.
In screen saver mode the screen is totally black. I have checked the cabling to the monitor to make sure it is not loose or faulty so it is not that. I was wondering if this is a virus or whether the screensaver programme is corrupted?