Computer switches itself off

[#1]

Possibly a short but more likely to be the current drawn from the PSU was a bit too high and possibly started to burn.
Glad to hear that the new PSU is making a difference so far

[#2]

might also want to invest in a good Zalman heatsink and fan.  My Athlon run at between 28 (idle) and 38deg (flat out Battlefield 2142 mayhem).  Some good thermal paste too.
Best supplier i know is Microdirect (http://www.microdirect.co.uk)
it's a good idea to strip and clean your system at least once per year, or when you do upgrades, like Video, Sound, HDD or memory.

[#3]

My desktop too started falling over after a new motherboard and extra ram was added. A black screen suddenly appeared and everything had gone! After a restart all was well... till the next time No warning, just OFF. It didn't appear to be a memory thing, it turned itself off even five minutes after start up. After a lot of head scratching, replacing memory, trying other boards, clearing dust etc, I found the reason. It's called Service Pack 2! Something in that pack clashed with my new build causing the crashes. I took it off and the machine has been perfect since. Worth a try.

[#4]

Hello Weejohn
I dont know how much or little you know about PC's so forgive me for asking this question.
When you changed motherboards did you reinstall windows? Have you changed motherboards and then loaded SP2 on top?