Dell latitude randomly turns off...

After helping others so much, decided I'd start asking some questions myself! ;)

I'm helping debug an old latitude c800 unit, and I'm stumped... looking for some brainstorming here.

The unit seems to be able to run a knoppix or ubuntu livecd without issue for lengthy periods (I'm about to set it up and leave it going for a few hours).  trying to install ubuntu, or XP (what I really wanted), eventually dies at some point during the install.  And 'dies' means 'just turns itself off at some random point'.

I have tried a different hard drive.  Guess it could be the controller on the motherboard, which would (obviously) stink.

I have tried swapping the freaking backup battery with a laptop that is a little younger, and shows no such issues.  No such luck, still exhibits same problem.  (That was my hope, as I've known desktops to exhibit similar wacky issues...).

I don't think it is a heat related issue, given that I can run livecd OSes for extended periods, but maybe they aren't taxing things the same level.  However, the unit does not heat up to the touch like many laptops, seems to be keeping its cool. ;)

I just tried a different SODIMM, and in the second slot instead of the first (yeah, changing two variables at once).

I've tried multiple AC adapters, btw.  Haven't gone too far down that path, just given that livecds work for a while (and certainly 'shift' the machine more reading a cd than touching the hard drive).  Could be some power-draw issue I guess.

I just booted knoppix livecd, had one of the fullscreen games on its main menu for about 10 minutes, then exited that and brought up a shell, started doing ls -R.  I got some cases where the hard drive made odd noises, but at no time did the machine just shut down.  But, the drive IS mounted in read-only mode -- which may eliminate certain issues.

XP Setup dies either during the drive formatting or just at the end (usually within the start of formatting though...).  Ubuntu install has gotten to various stages -- one time it died during the initial copy, one time it actually made it into the reboot-from-hard-drive and was unpacking/installing packages.  In any case, no OS installation has survived -- it might be minutes, it might be an hour, but it eventually just turns off randomly.

Dead motherboard?  Dead hard-drive controller?  Something with power?  Is this just spare parts at this point?  (I'll double the points if someone actually can lead me to get it working, versus proving it's dead... ;) ).