"New" A4000 power problem

[pedro7]

What happens when it shuts off, does everything lose power ie fans/drives spin down completely, LEDs off? Have you tried monitoring the PSU voltages as this happens?
If you tested the PSU without anything connected to it might be that it gives the correct voltages when no load is on it but when a current is drawn the voltages drop. I'd test it with another PSU if you can.

[pedro7]

Hmm, strange.
Maybe there is a short circuit between ground and one of the power lines on the circuit board. This could explain the known good PSU lasting longer, the other one could have been damaged by repeated short circuiting. I'd suggest using a replacable AT/ATX PSU for any more tests.
Check with a multimeter for any shorts between the power lines. I think I read somewhere that the a4000 can have problems with the pins from the power connector on the circuit board underside touching the case and shorting. Is there any physical damage to the board anywhere? Bent pins? Pins touching in the zorro connectors? fried components or heat damage from shorting?
It can't hurt to take out the zorro backplane and everything else non-essential and reseat chips and ram.