Is this a machine with a reseatable processor? I remember a while back, about a month afater my dad got his pismo, it would kernel panic very early in boot up, in almost every boot up. I reaseated the processor card and its been happy for more than a year now. So, you might want to try making sure its well seated, and that all the contacts look clean.

If its not a reseatable one, I would guess RAM problems.
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iMac G5, Powerbook G3/333, Beige G3MT, PM 6200CD, RS/6000