I am going to guess you pulled the battery out too? If not, yank it. Probably not the cause but ya gotta eliminate everything. Next do a power management reset:

http://support.apple.com/kb/ht1431

After that, its onto hardware. It could be the power/charging circuit. Which might be on a sub-card.

Edit: Google Gods say is could be a bad PRAM battery? When they go bad for me, its usually dead clock time, etc. I know some Macs can be fussy, but I thought older 68ks. Either way, its something to look at too.