2008 Mac Pro random power cut+boot issues

Question

Hello everyone,
I'd like to describe a problem I've been having and see if it tweaks any ideas as to what the issue might be.
In 2008, I purchased a Mac Pro 2x2.8. It's got 4gb ram (2x 1gb Hynix and 2x 1gb Kingston), 320gb, 1tb and 160gb sata's and a ATI 4870 graphics card. Up until about 3 months ago, everything has been dandy - it's the best computer I've ever had, however recently I've been having some intermittent problems that may be related, or separate.
Problem 1: In the morning when I turn it on, on some occasions it will power on for 5 or so seconds, then power right back off again. Sometimes this happens after 5, 10 or even 20 seconds (right in the middle of OS X or Windows boot). The only thing I note is that if it can get as far as the boot selector (hold alt at boot) screen, it won't cut out at that point, only sometimes after I wait a moment then select a boot option. This can sometimes happen 10 or 20 times in a row before it will eventually turn on and function normally.
Problem 2: Random power cut off, sometimes at idle, sometimes while browsing, or using any other app. I can't pin it on any one particular function. I know it's not a temperature problem, I have temp monitors on my menu bar and as far as I can see it's been fairly stable in that regard. Most of the time, I can power up the computer again but very occasionally it will revert to what I described in problem 1.
I have run the apple hardware test, with no errors being produced, and have had the local apple reseller/service centre take a look, with their tech reporting no issues. (He referred to something called ARD being run on a cycle of 20 or 30 times with nothing indicating a problem) I've taken off every non essential piece of hardware, tried with only the stock ram - and the newer ram to no avail of my issues. The only thing that I can still suspect myself is a power issue, however there is another PC in the room that has not had any issues what-so-ever.
Is there anything anyone could suggest I try to pinpoint this issue? It's sending me insane.
Thanks in advance for any help!

Answer 1

I found your post after dropping my G6 off at the Genius Bar - for precisely the same reasons. I've had this issue for the past 6 months on my (early) 2008 dual quad core. A month ago, the power supply entirely died and Apple replaced it. Now the same issue has returned and the computer restarts multiple times on boot up, at different points, sometimes as long as 15-20 minutes after booting up, when everything seems normal. At this point, two people at Apple have given me two different causes. 1) The person on the support phone said it has to be the power controller on the logic board. 2) The person who took in the Mac said it has to be the APS UPS (model ES 750) because the G6 has a 900 watt power supply and the UPS is only rated at 450 watts (750 volt amps). I had already thought of this as a possible cause and had purchased a power consumption indicator called Kill-A-Watt. Guess what? The G6 consumes at most 120 VA. In any case, the UPS fuse would have blown at overload. I don't feel like springing for a 1750 VA UPS unit just on the off chance that this might be the cause. This is also driving me up the wall. Additional info much appreciated if anybody cares to make educated guesses.