No sleep, no hibernation, no power reduction, it just turns off as if I held down the reset button. Actually, though, I have a battery powered mouse that hibernates after a few minutes of inactivity. I have to press a mouse button to get it working again. Maybe that is related to my problem.
I'm trying the software recommended by Briton30 that moves the mouse pointer 5 pixels at set intervals. I set the interval at 5 minutes and so far so good.
Thanks for all the help!