Automatically shuts down when inactive or when put to sleep

My Macbook has a condition where if it is inactive for more than a minute or two, it puts a dialog up asking if I wish to restart, sleep, shutdown or cancel. If this is ignored for more than about 15 seconds it shuts off as though it lost all power. this occurs in the login window as well as in any and all user accounts. I have reformatted the internal HD and reinstalled the OS from the DVD and have to move my finger on the trackpad to allow it to complete the installs- Might you have any experience with such behavior?

the machine functions in target disk mode without any problem as well.