Interesting site, though I do not find any problem descriptions similar to my own. I have a pismo 400 with upgraded RAM and HD, to make OSX usable. The problem: after 2-3 minutes the screen goes black, no image visible even under a bright light, and the i/o freezes up.
After a back button reset, it powers down fully and can be started up again. This occurs in OS9 or OSX the same, I've swapped/tested RAM, have a good positive seat of the cpu board. Thinking it was heat getting to the cpu, I reapplied thermal paste at cpu and heat sink, though I can't remeber the last time my fan went on or my battery held a charge.