[Hampshire] help needed to diagnose computer problem

My main system suddenly powered itself off yesterday whilst I
was attempting to browse a website. It restarted again but would
not stay on. I'd hear a click somewhere in the box and it would power
off and immediately try to restart. The length of time it stayed
powered up varied, even to the point I could reload X on a couple of
occasions.

I tried eliminating possible causes by disconnecting/removing everything
possible and disabling everything in the bios but could not come to
any real conclusion as to cause.

The hard drive is working perfectly in another system although it did
have lots of errors, after all the stopping and starting, requiring a
manual fsck so I think it is time to back up the personal data
on it in preparation for swapping over to a sata drive.

I suspect either the motherboard (a fairly elderly ASRock branded board)
or the power supply but need a bit more confidence as to cause before I
start ordering replacements for either.

So far I have only found PC World & Maplins in Yeovil for obtaining
parts so will have to use an online vendor I think.

Suggestions as to likely cause of powering off are welcome.

--
John Lewis
using Debian Sid with windowmaker for a nicer desktop