Rev. B iMac Unexpectantly Shutting Down

Since June of 05, my iMac has run almost continually with out a hitch. I get back from a 9 day vacation and see that my computer has shut itself down (I know it was working, because I was able to iChat the computer from where I was the night before returning).
Since Sunday, my computer can not stay on. It will shut down as soon as the login screen appears, 20 minutes into a session, 2 hours into a session, or longer ((overnight, sometime between lunch time and dinner time when I'm not at the house))
I have read and done the suggested 'fix' from Apple with no luck:
http://docs.info.apple.com/article.html?artnum=86817
I thought it might be a RAM problem, so I took out the 1GB stick I purchased last year. It seemed to do the trick, and worked for as long I was working with it last night, then I come down this morning, and the machine had shut itself off. So I'm thinking it wasn't the RAM.
Where should I go and look next to isolate the problem.
My Mac is not a Revision A that the "known recall" issue with the mid-plane and or power source. However, it is in the upper range of one of those serial number ranges.
Any help would be greatly appreciated.
Thank you.
CP Staley

Not to sound like a lecture, but who knows that happened when you left your Mac unattended for 9 days. No matter what, I always am sure to shut down and unplug my computers when I am gone for more than 2-3 days. It is just the smart thing to do, even if you have a UPS. There is no good reason to leave your computer running when there is no chance that you will be using it since you are not home.
Take a look at this Apple FAQ and see if anything there helps. I doubt that it will, but Apple will ask you what you have done and try to walk you though these steps anyway, so do it before you call them so you can say you tried.
I think that you have the same problem I did with a bad PSU and/or midplane. It certainly sounds like the same issues that led to my needing those replaced.
Try the steps above and then call Apple if they do not work. They will ask you for a credit card since it is outside of the 90 days tech support, but if you insist that it is a hardware issue, they will walk you through a few things to verify that and autorize you to take it in for repairs.

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