IMac G5 shuts off and won't restart

Mac suddenly shuts off, and won't restart. Begins to boot, but shuts down again, sometimes after only a few seconds and before the chime, sometimes after 30 seconds, sometimes after several minutes. After many attempts, can sometimes get restarted. If I hold in the power button, I get a long tone, and then sometimes get a gray screen with Linux style text, and if I type mac-boot as prompted, it will usually reboot. Then its good for a day, or several days but eventually, will go off and I can't reliably get back to that gray screen with text.
Not a techie, or Linux fluent...just got there by accident.
Is there something I can do / reset / or replace to fix this? (Yes, I know the computer's old, but I'm not ready to go to the Intel chip and the expense of repurchasing all my software yet!!)
Thanks for any help.

I have the same 1.8 GHz iMAC G5, and had the same problem of the computer shutting off abruptly, and then not starting up - shutting down after the chime, or sometime during the start-up process (I don't know about the gray screen - never tried that). Eventually the computer just wouldn't turn on at all - now it's dead.
I found this Apple support article recently, which shows how to diagnose a dead iMac G5, to see whether the computer has a bad power supply or a bad logic board. It might be helpful - in my case, it points to a bad logic board. Maybe both are bad (?). The procedure is for a dead computer, but maybe it would show something in your case too.
http://support.apple.com/kb/HT2173
Good luck. I don't how much it would cost to put in a new logic board or power supply. I love the monitor on the G5 iMac (beautiful non-glossy), and I wish I knew of a way to use the monitor by itself.

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