I have two HDDs, so disconnected both and tried booting the machine. Unfortunately, there was no change in behavior. I just extrapolated your theory a bit and started trying to boot with each device disconnected - DVD reader, writer, wireless network card and display card (I connected the monitor to the onboard VGA port) in that order. The boot/restart became normal the once the display card (NVidia GeForce 8600 GT) was unplugged.
I've been running the machine since morning with onboard video and except for the terrible resolution everything else is perfect - no boot/restart problems.
The big question - could it really be the display card at fault or am I looking at a possible motherboard issue? I ask this because with the card plugged in, (and once the machine booted) I never noticed any problems with the display. Thanks again and look forward to hearing from you.