[Hampshire] Random shutdowns (Ubuntu 7.04)

Hi all

I'm suffering an apparently random shutdown which could be to do with many possibilities - can someone check my logic. The machine is very new - and highly enough specified (Sempron, 768Mb RAM) but has exhibited a couple of problems. It's now running Ubuntu 7.04 (recently upgraded from 6.06 to 6.10, then to 7.04) but none of these recent upgrades have causes these problems. [On a related note - full marks to Ubuntu for their upgrades - no problems whatsoever and it even kept my large fonts which I promise is a real Godsend]. Anyway, back to the plot:

1) When not in use, under no load, it sometimes powers down without warning.
2) When not in use, again under no load, it has started to make a loud whining sound.

As far as 2) goes, I've disabled the CPU temp warning and that seems to have fixed the sound. Obviously this points to the thing overheating but the fan is working and there's no dust, and all the expansion slots are uncovered so there shoult be plenty of ventilation. I'm thinking the only solution here is to buy a separate fan. Am I right here? I should point out it does have a couple of quite old hard drives (one of which has the "click of death") - but see below - I don't think it's the HDDs.

As far as 1) goes, I've looked in power management and I've not asked it to shut down at any point. I've also looked in the user log (/var/log/user.log) and I get the interesting error "GConf server is not in use, shutting down" which seemed to occur around the time it shut down on its own (I wasn't in the room at the time). I've found a couple or (unanswered) articles thanks to googling for that error - but not found a solution. Can anyone suggest a suitable line of enquiry?

Last thoughts - if I need to buy a new fan - never having done so in my life - what sort of things do I look for - should I buy one that connects to the motherboard, or one that goes straight to the power supply?

Cheers
Rob