Why does my iMac power off?

After 5 or 10 minutes of use, I hear a "ping" like a the sound of a light bulb burning out, the screen goes black and the iMac has turned itself off. I can turn it back on with the power button, but it just happens again.
Is this what is meant by "crashing?"
Where do I begin to diagnose?
Thanks.

Posting here instead of its own thread because I am having the same problem on 20'' iMac G5 (pre-integrated iSight, purchased in August 2005) running Mac OS X 10.4.11.
It powers off unexpectedly (not normal shutdown--sudden interrupt as if plug was pulled). It doesn't happen after any set period of time--sometimes shortly after startup, sometimes after it's been on for hours.
I tried resetting the PMU as per the Support directions. No change, although I wasn't sure if I actually did it correctly--the directions were simple, but there was no confirmation tone or anything like that.
I tried resetting PRAM. No observable changes.
I don't think it's a heat issue since the computer has even powered itself down like this while asleep!
I suspect it's a hardware issue, but before I take it in somewhere (it's still covered by Applecare), I though I had better ask you knowledgeable folks if there were any other steps I could do on my own. It'd be a bad time of year to be without my Mac for even a few...days? Weeks? How long might this fix take? =(
Thanks in advance for any insight you can provide
P.S. If I should go ahead and give this its own thread instead, let me know

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