How can I get I-MAC to stay on?

Hi Experts  I have a 2004 version of an I-MAC G-5  1.6Ghz that starts up fine, arrives at the desktop fine,  allows me to work for a while, (90-129 secs) then it shuts itself off. NO Variation.

Capacitors all look like new.  Tried different memory.  Other than that, I do not know what to do, to find the cause of this behavior. On a scale of 1-100, I am less than a one on a MAC machine.

QUESTION IS:----What is causing this behavior and what is neccessary to repair it?

Thank you in advance,
Donnie616

PS: I have an identical machine (to learn MAC) in the basement that works fine, but it contains 4-5 severely leaking capacitors and I was thinking of trying to swap Motherboards.  Unless of course the MB(Board Logic) is what is wrong with this one.  Anyways, that is why I need the Experts.  
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Donnie616Asked:
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strungCommented:
If you are up to that type of surgery, I guess it couldn't hurt.

Have you tried a safe boot while holding down the shift key?

Or here is an interesting experiment -- will it stay on in Firewire Target Disk Mode (i.e. booting holding down the t key)? If it displays the same symptoms in FTD mode, it is almost certainly a hardware problem.
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strungCommented:
Check the Energy Saver prefs pane just to make sure you haven't set a short shutdown timer.

Also to eliminate a hardware problem, boot from a system CD and see if it exhibits the same problem. If it runs fine from a CD, then the problem is likely software.

It sounds to me a like it might be a heat problem. Is the computer properly ventilated, insides not covered in dust and fans running properly?
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strungCommented:
Also download and install this freeware temperature monitor to check the temperatures on your computer:  http://www.versiontracker.com/dyn/moreinfo/macosx/19994
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Donnie616Author Commented:
Thanks.  No CD available.  PC does not stay on long enough for me to navigate through Energy saver prefs.
It stays on no longer than 90 secs, OR if I use the mouse, no more than 2 or 3 mouse clicks before it goes off.  No time to do much of anything.

I have read in EE knowledge base that PSUs were extremely problematic in this model.  

Would you rerccommend that I swap PSU from the better MAC to this one as a trouble shooting effort?  Thanks
D616
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Donnie616Author Commented:
Thanks for the couple of tricks you taught me.  FTD for example.  Holding shift key did nothing, as it shut down while I was doing it.  "Surgery" seemed to work.  doe not matter, because now I need admin's password to do anything anyway.  I do not have that and cannot get it.  One day when i learn about MACs, I will obtain a disk and install the OS over again.

Thanks again
Donnie616
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Donnie616Author Commented:
Thanks for you help. I am surprised nobody else answered because this exact topic is all over the net.
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strungCommented:
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