Ally,
I'm no guru but will try to step through some explanations and troubleshooting steps.
The motherboard, cpu chip, memory and power could all contribute to spontaneous shutdown. I would go ahead with your PSU replacement to eliminate one "failure vector". I would encourage you to go with a higher wattage PSU when you replace it -- for investment protection (when you upgrade your mobo/cpu). When going for a replacement try to go with a "quality" PS. My personal favorites are thermaltake, antec, vantech, or enermax -- your mileage may vary. The bottom line is to add up your systems wattage requirements and make sure your PSU can handle it. [see table at bottom]
Let's talk about some troubleshooting steps you can take to try to nail down this problem. I would download the following (good tools to have anyway):
1/ Sandra
http://www.sisoftware.co.uk/ (sys info -- very good)
2/ aida32
http://www.aida32.hu/aida32-download.php (another sys info)
2/ MBM5
http://mbm.livewiredev.com/ >downloads (look to see if your mobo is supported -- mobo info from top 2 proggies)
Power:
Your cpu needs to have a steady power supply (ie it shouldn't move around or flucuate wildly) this is true for vcore (most likely around 1.7 v) and your +/-12, +/-5 and +3 voltages. If vcore is lower than the spec for your chip, replacing the power supply may fix it, or you may have to replace your mobo (the transformers on your mobo may be faulty) I asked about load vs idle because both power and heat can be a factor under load (the fans will draw more power as they need to spin faster to cool your system (unless they are -always on type fans-- running at highest rpm). The power draw from the fans is not likely to be the problem, but if they are not getting enough power to spin fast enough heat could be the problem. If your (v)core voltage is either too high or too low you can experience all sorts of unstable behavior. If vcore is too low (below spec) and your bios supports it -- you MIGHT want to try boosting vcore as a workaround. The downside of this is heat, which means you will need to monitor the temp of the cpu. (see above or other tools (bios?) to do this)
Heat
We need to see if your system is overheating. Even though you said it happens in a short time, if the cpu isn't being cooled properly it can heat up/shut down very fast. Check to see if your bios has temp/power monitoring (or if your mobo came with software to do so in windows) or try to get MBM5 working so you can monitor both power and heat on your system. AMD cpus tend to run a little hotter than Intel chips. AMD and Intel both have maximum temperature ratings for their CPUs listed around 80C. If your CPU gets this hot, you’ve got some serious problems. My new system (AMD 64) is running at ~45-47C at idle -- I haven't really stressed tested it yet under load. Best practice to to keep the cpu between 45-55C. This doesn't not mean your PC will not operate at 60C, but high heat temps for extended periods of time will reduce the life of the CPU. You may want to check the "mating" between your HSF (heatsink) and the CPU. If your PC came with stock thermal grease you may want to replace/renew it. I suggest Artic Silver 5 as a replacement. (Make sure to get the old grease/compond off before you put more on - I use Goof off for this) If your CPU came with the stock HSF you may want to replace/upgrade it. (I just upgraded to the Volcano 12 from Thermaltake and dropped my cpu temp by about 5 degrees over the stock HSF -- at ~3800 rpm)
Mobo:
Your motherboard could be faulty and not supply the correct voltage (esp vcore) to your CPU. Look at bios/other monitoring tools to check this.
Memory:
Bad memory can cause instability and shutdowns. Look on the net for a memory test and run it.
Processor:
Run stress test/diags on your processor. It could be faulty.
Power usage table:
(these are rough estimates -- research your components to make sure )
AGP Video Card 30 – 50W
Average PCI Card 5 – 10W
10/100 NIC 4W
SCSI Controller PCI Card 20W
Floppy Drive 5W
CD-ROM 10 – 25W
DVD-ROM 10 – 25W
CD-RW 10 – 25W
7200rpm IDE Hard Drive 5 – 20W
10,000rpm SCSI Drive 10 – 40W
Case/CPU Fans 3W
Motherboard (w/o CPU or RAM) 25 – 40W
RAM 8W per 128MB
Pentium III Processor 38W
Pentium 4 Processor 70-90W
AMD Athlon Processor 70-100W
USB-Devices 5.00 W
IEEE 1394 8.00 W
Keyboard 1.25 W
Mouse 1.25 W
Another resource (which I hope doesn't give VASTLY different numbers) is:
http://takaman.jp/psu_calc.html?english
***For overall power supply wattage, add the requirement for each device in your system, then multiply by 1.8. (The multiplier takes into account that today’s systems draw disproportionally on the +12V output. Furthermore, power supplies are more efficient and reliable when loaded to 30% - 70% of maximum capacity.)***
In addition, it is important to keep in mind not to just total up the watts you need and buy the power supply. If you need 350 watts, do not get a 350W power supply. If the computer has a 350W PSU and 350W is needed then the PSU has to run at full power the whole time and would over heat and die very quickly. Power supplies are much more efficient when not fully loaded. Also read carefully what the PSU vendor is saying relative to peak load. My 480 watt PSU is rated at 550w peak load. This is most important during startup (almost all electronic devices will draw alot of power right at startup, then settle down to their operating draw).
-chanzia