hmm... curious.
I opened it up, blew out all the dust, left it open and started it up.
It shut down once, then started up again and has stayed on for over 30 minutes. (the longest it's been on in 3 days). I'm gonna leave it open for awhile & see if it shuts down again. There's no loose wires or connections. No circuit boards wrongly seated etc etc...

So... it seems like I've got it working, but now the recovery discs are corrupted so I've got no working copy of WindowsXP. (the recovery process in safe mode won't complete. it gets to disc #4 and aborts because one of the files is corrupt)

I guess the next step is to see if I can get a pirated copy of WindowsXP (there ain't no way I'm paying for it) or waste money on Vista.