I have seen a few of these where the problem was the Power Supply. Look on the 'Apple' site with the serial number of your power supply and then try to swap it out with a rebuilt unit. You can also check the outputs for their proper voltages, all listed on the apple site under 'troubleshooting'. There was a problem reported with a couple of components, that as soon as they heated up, they would short out and the PSU would shut down as a result.
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According to your display of error messages, this could also be a 'wanky' memory chip that is very voltage sensitive. I would get a good D.M.M. and check the PSU first!
I have seen a few of these where the problem was the Power Supply. Look on the 'Apple' site with the serial number of your power supply and then try to swap it out with a rebuilt unit. You can also check the outputs for their proper voltages, all listed on the apple site under 'troubleshooting'. There was a problem reported with a couple of components, that as soon as they heated up, they would short out and the PSU would shut down as a result.