I have a custom build computer:
CASE: Chieftec DA-01BD Blue Dragon (Full Tower), measuring 470x 205x 670mm(DxWxH).
CPU: AMD Athlon XP3000+
RAM: 2x Crucial 512MB (1GB total)
VIDEO CARD: Sapphire ATI Radeon 9700 Pro

Around a year ago (April '04) my computer started shutting down during graphic-demanding jobs, such as games. I ran a virus check, but could not think why it was happening. I was told maybe the CPU was overheating, and ran "Motherboard Monitor 5", and I have a resting CPU temp. of around 51-54, with max around 62. However, the problem would only occur during intensive graphic jobs, not during genereal demanding jobs.

It was then suggested that I take the side of my case off to aid air flow, and cool the inside down. So I did, and it seemed to work - my computer stopped turning itself off . . . until now (April 05) roughly a year later. Again, only during games, though it happened twice during a relatively hot day (the room was quite stuffy) when it had been on all day and I was runnig quite a few applications. The problem is always the same, I start a game up, it'll give me 10, maybe 20 minutes before the power just cuts out.

I can only think of 3 possible causes:
1.) The graphics card is malfunctioning.
2.) The CPU is overheating and causing the computer to cut out.
3.) The Power Supply is malfunctioning.

Solution wise, I can only think to replace the CPU, or PSU, or buy a new case, as my current one (I accidentally ordered it) is far too big.

Does anyone have any ideas as to what could be the cause of the problem? Any help would be much appreciated!

It does sound like an overheating problem. Since you have been running with the side off of your case I would suggest a good cleaning.
Get a can off compressed air and a soft brush and clean your heatsink and fan on your cpu.Clean all your case fans and your graphics card fan and heatsink and anything else where dust builds up.

Does your BIOS or do you have software that will shut down your computer at certain temp thresholds?

hmmmm. Compressed Air? Where would I buy one of those, just a DIY shop?

How do I tell if my BIOS settings shut it down? I.E. do you know which menu it would be under, or are all BIOS setups different? I'm not aware of any software that sets a temp. threshold.

thanks for the suggestions!

That CPU should not shut down the system at 62C.

I've accidently run one at 82C without the symptoms you describe.

HELP!

I bought a new case complete with new power supply, cleaned all my components and have transferred them to my new case, it started up A okay, then after about 5 minutes, I was in Unreal T 2004 (just the options screen) and it froze for a couple of seconds and turned itself off AGAIN! I am now at a COMPLETE loss to what could be causing this. It was running at about 48 degrees a couple minutes after start-up so I don't think it can be the temperature.
I did notice when transferring components that the CPU fan had been connected to the power-fan socket on the motherboard instead of the CPU-fan which I corrected in the new setup. There is no setting in the bios to turn it off at a certain temp.

You don't mention what motherboard you have.
Most newer boards have the shut down option.
You mention MBM5.Check the settings in that program. It has temp limit settings. It has been some time since I used it so I don't remember if it just alarms or has a shut down option.

The plot thickens . . .
So I replaced the PSU and case. It hardly gets hot in my room, and there is a case fan as well as CPU fan and video card fan. Now however, not only does my computer still shut down (as in a sudden loss of power, not a Windows "Shut Down" as such) when I play games - which makes me think it's video card related - but also when it's just idling. Literally, I'll browse the web, listen to some music, watch a film, do some work. Then go downstairs, leaving the computer completely idle, come back . . . and it'll have turned off!

I have no shut down settings in Windows, and MBM5 isn't even open. I am at a complete loss and am worried that soon either the hard drive will become severely damaged or the whole thing will just blow up.

My motherboard is an "Asus A7N8X-E Deluxe nForce2" - could that be the problem? But I don't see why it would be okay with general work, but then shut down when it idles.

Any help or ideas would be much appreciated!