I have a G4 500MHz 512 that is giving me problems so serious, that it will not even boot up now. I had a tech look at it after it first started crashing and he recommended a new HD, so I bought a 120 gig and and transferred everything over to that from the original 27 gig HD. Nope, not the problem. Another tech ran diagnostics and found nothing wrong with it. The third tech (Apple authorized) ran diagnostics and found directory issues, which he fixed. He pronounced it in good health and sent me on my way. It promptly crashed when I started it up. One thing I noticed was that at night I would turn off power to the machine and in the morning it seemed to work fine for a few hours. I thought it was a discharging thing. I read in another forum that a person with a very similar issue took the battery out and replaced it and his problem completely disappeared. AHA! I replaced the battery and it ran fine for 3 days. Monday morning I turn it on after a weekend of no power, completely discharged and it freezes. I cannot boot up from the CD. Most of the time, the HDs don't engage. After sitting for awhile, they will spin and it locks up, sometimes while in the grey screen, sometimes in the blue screen, and sometimes just at the point of being open. After that, the HDs won't engage for a long time. When I replaced the battery, I zapped the PRAM according to the Apple website's directions. I'm at my wits end. Can anyone offer me any advice?