i never had one of those power mac's - but it looks like a capacitor or a power supply issue - had a similar thing on my hp notebook - in my case it was a capacitor problem. but i've also seen problems with empty pram batteries!
i would start with measuring the PRAM battery, if that won't help - measure the PSU - the pinout (and possible conversion of an atx psu can be found [http://www.insanelymac.com/forum/lofiversion/index.php/t105113.html|here (CLICK)]) with a multimeter