Computer powering down

I am trying to fix a friends, rater old, compaq 5000.

The problem is that it simply keeps powering off ... sometimes it can go for
5 minutes and sometimes only a few seconds...

I am sure its not a heat issue, since I have refitted the cooler with artic
silver 3...and I have experience in installing these so there shouldn't be a
problem (also tried two times to be sure)

its not the ram or PCI cards, harddisks or cd-rom drives or anything.

This happens with everything but the PSU and motherboard
disconnected....also its not the PSU...I have tried several different
ones...units that I know are working.

So its most likely the motherboard...but what can cause this...and can I do
anything but replace it?

 

www.motherboardrepair.com

is a good place to start/or finish

 

Unless you can find some obscure power saving setting in the BIOS - I would
suggest that you have found the solution to the problem and replacing it
would be the easy option.