I would try a simple test....if you have another PSU laying around or in another puter (could be a friends) power your system up wiht that PSU. If it routinely powers on and off it's the PSU. If you experience the same problem with a different PSU you are having voltage issues with the mainnboard(bad or failing system capacitors). If you have a digital voltmeter around I would check the voltage to the greeen primary lead on your twenty four/twenty pin mainboard connector and see if you have resting 5 volt power to your mainboard( If that lead is routinely 10% under your mainboard needs to become a frisbee). If you don't know what I'm talking about here then jsut do the PSU swap and that will give you a good starting point.
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