Random system shutdown/reboots

This has to do with an old laptop I had a few months back.
Specs:
ABS Mayhem G4
Pentium M 2.13ghz
2gb 533 RAM
100gb HD
Geforce Go 6600

Its battery was long dead and for about a year I was just using a powersupply to give life to the old laptop. Well one day on this latest deployment it rebooted for no reason on me.
-Fast forward a day or two and its rebooting more often on me.
-Fast forward a day or two and it wont even boot up windows without rebooting 3-4 times and once windows loads it reboots at random maybe 5 minutes into a session or 2 hours into one.
-Fast forward a week and its a pain just to get windows to load. It reboots or shutsdown entirely throughout the load process. Sometimes at the intel splash screen, sometimes it makes it to the windows splash screen, sometimes it makes it almost into windows and shutsdown
-Finally a day or two after that I can no longer even get it to boot into windows.
-A day later all it will do is power cycle and wont even make it to any splash screen.

It was a pretty rugged laptop. Its gone to Iraq twice for me and come back unscathed. I used it frequently at home between deployments but when I started moving it around for this third one it started giving me this problem.

I've since scrapped it for a Dell XPS 1710 but I wish I knew what really caused its demise.

That was one of my thoughts as well but haven't had a good way to test it out or not. Don't have a battery that will work with the laptop and I have to wait until I'm home again to try and contact ABS to see about getting a replacement power supply for it.