IMAC 27" goes black, won't run or boot up for hours. Power supply?

and then suddenly be able to boot. But when you shut it back down, it won't boot again. Is this a power supply issue? I've cleaned it and tried multiple power outlets around my house, but the only cure seems to be to sit for a few hours. I use it with a Thunderbolt monitor on the side, but it doesn't seem to make any difference whether it's part of the equation or not.

Hello ... I start to have the exactly same issue with my 21" after a shutdown or sleep mode ... ... I have to wait some time until I can boot again. If I try to boot after 5m or so I can ear the Hard drive and fans but it doesn't start (no screen, no startup sound or keyboard access). Any sugestion?
Thanks in advanced,
Jorge
PS: I have already OS X 10.8.1 (12B19) version and this is a ( late 2009 iMac 21" 3.06 GHz Intel Core 2 Duo)

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