Sony desktop

I have a customer who brought his computer in a few weeks ago with shut down problems,what I mean by this is while the computer is running it just turns off.Then the computer wont turn back on.
Ive check the bios(power managment and disabled)
Ive disabled power managment in the control pannel.Ive checked all components in the computer,
Ive even taken the power supply apart and cleaned all the caked on dust out...this computer has been in my shop for the 4th time now.Just when I think the problem is solved ,it's back...the computer is a sony desktop,model pcv-r528ds.
The shutdown occurs randomly.Ive had no problems runninng this computer for 2days straight,shutting it down and restarting,then out of the blue it just shuts down and wont restart(no power)for a 1/2 hour or so.Does any body have any Ideas to what it could possibly be?

If it shuts down and won't come on immediately, but will restart after a while it may be a heat issue.

thats what I thought,and it probable was.. that is why I took the power supply apart and cleaned it..
before i cleaned it the power supply would get really hot,now it stays cool..anybody else have any suggestions?

Disable the acpi if you can. These chips go bad, and give you strange stuff. Is this sony stock?