IMac shutting down spontaneously!

Like the title says, my iMac will shut down all by itself, I think it is associated with getting really hot as it only happens when the computer has been running for a long time, like say 10 hours or more. Details- after this happens, the computer won't start up, is completely unresponsive, unless I unplug the power source cable and plug it back in. This is true without exception. Surely this means something.
Did the Disk Utility and the fsck -fy trick upon starting up (after this happened) while holding command (apple key) and S. Dunno what those things or that mode is called. They report back with no complaints of any kind, everything is OK.
Can anyone tell me what the issue might be? I love this computer and having to leave it would be a tearful experience!
feel free to ask if you need to ask about anymore details.
Thanks in advance for your helpful responses
Agamemnon

Thanks to all for the input, I solved my problem completely and my beautiful 20" flatscreen iMac is running like new. I'm deeply, deeply embarrassed to say that the problem was solved by cleaning out the air intake vents on the bottom of the base of the computer. I vacuum regularly around it, but when I got a good look, each one of the slots was filled with what looked like a pianohammer of felt. I vacuumed it out by sticking the very edge over the edge of the table, and vacuuming while poking.disloging the gunk with a bamboo skewer. All with the machine off, of course. I slowly rotated the machine, continuing this process until I was back where I started.
Not only does it not shutdown on its own, it's quieter than it has been in years!!!!! It runs MUCH cooler as well. I shudder when I remember how hot everything got. discs out of the drive were nearly unhandleable!
So, happy end, moral of the story- it pays to clean, or, GRIME does not pay!
G4 800 mhz iMac Mac OS X (10.4.5) 5th gen iPod

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Hello there, TheCapGuy.
The following Knowledge Base article is a great resource for steps in troubleshooting spontaneous restarts:
OS X: When your computer spontaneously restarts or displays "Your computer restarted because of a problem."
http://support.apple.com/kb/TS4636
Thanks for reaching out to Apple Support Communities.
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Best Regards,
Jeremy

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IMac shutting down spontaneously!

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