Imac won't start unless unplugged for hours

OK, a little history (I'm not sure if it's related, but here it is):
About 2 months ago, I started having trouble getting the computer to wake from sleep. Then, it started having kernel panics. Then, it wouldn't consistently start, and finally would give me the 3 beeps on start up, indicating the RAM was bad. So now, with the offending RAM removed, here are the symptoms:
The iMac (24", late 2007) will sometimes spontaneously shut down after a couple of hours. It cant' be restarted unless I leave it unplugged for several hours. I thought it might be a heat issue, so I downloaded smc fan control, ran them up around 2000-2500, and put iStat Pro on. The temps all stayed below 55C. When I go to turn it on, if shorter than the several hours, I can hear the optical drive make a little sound (I think that's what it is) and then absolutely nothing else happens. Also, once it shuts down, if I leave it plugged in for those several hours, it still won't start. Also of note is that I am not doing anything intense - just some email and Word writing. Anyone else experience this and have any solutions? Or other questions to help me diagnose it?
TIA.

Try running the Apple Hardware Test to see what it throws up, you might also boot from your install disc and use Disk Utility
1. Insert the Mac OS X Install disc that came with your computer, then restart the computer while holding the C key.
2. When your computer finishes starting up from the disc, choose Disk Utility from the Installer menu. (In Mac OS X 10.4 or later, you must select your language first.)
Important: Do not click Continue in the first screen of the Installer. If you do, you must restart from the disc again to access Disk Utility.
3. Click the First Aid tab.
4. Click the disclosure triangle to the left of the hard drive icon to display the names of your hard disk volumes and partitions.
5. Select your Mac OS X volume.
6. Click Repair. Disk Utility checks and repairs the disk.
However, it does sound like a power supply problem, look in User>Applications>Utilities>Console and check the system log for any notations about temperatures.

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    Message was edited by: ChrisRR

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    Why reward points?(Quoted from Discussions Terms of Use.)
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