Need help finding source of computer shutdowns

I have been having incredible difficulty with my computer since I first built it two years ago, and replacement of parts up until now has not fixed it. I am one step short of just bringing it in and paying $100 to have the problem diagnosed, but this forum has never failed me in the past- so I want to try asking for help.

Here's my basic computer specs (I know there is a file I can get with details on my system, but I forget how to- if you need it, let me know, and I'll get it)

Motherboard- Soyo Dragon KT400 Platinum
Processor- AMD Athlon XP 3000+ (333 mhz fsb)
RAM- 2x500mb 400mhz dual channel DDR /w ultra cooling copper heat spreader on each
Graphics card- Radeon 9500 128 megs VRAM
Western Digital WD800JB (80 gig 7500 RPM (whatever the normal RPM is, that's it))
DVD drive- Sony DVD RW DRU-530A
Heatsink used- ThermalRight SI-97 with Vantec Tornado 92mm fan and Arctic Silver 5 thermal conductor paste used
Sound Card- SB Audigy 2
OS- Windows XP SP2
Antivirus and firewall- Norton Systemworks


*takes breath* Whew, okay, that should be a good summary. Here's the problem- sounds simple, but it's hard as hell to figure out. My computer simply restarts- one short beep and it restarts, often I have to turn it off and on again manually as it will just sit blank for a bit, but eventually I think it restarts on its own. This happens when 1) I am web-browsing, sometimes at certain webpages it will just restart. 2) Playing more recent games. Diablo II it never resets on... playing more recent ones, however, especially ones that utilize the power I bought it for, it will regularly restart. Particularly strong 3D games like Rome: Total War. Any 3D game, you name it. 3) It used to happen with burning DVDs when I used my previous processor, AMD AThlon 2500+ but doesn't happen so much now. 4) Almost always resets when I do a full virus scan, and used to reset when I installed large games on my hard disk before I upgraded the processor. Now, just virus- I put the computer in safe mode, and (after rebooting a few times trying to start safe mode) it usually works. My computer is, according to Norton Systemworks, completely virus free, btw.

Here's what I've replaced: I started with an AMD Athlon 2500+ with the HS/Fan that came with it, and upgraded to AMD Athlon 3000+ 333 mhz with my own fan/heatsink. The fan RPM is over 5000, which is good from what I know. The case side has little in the way of air holes, but there is a 92 mm fan on back sucking air in. 400 watt power supply. Due to driver problems I didn't want to pull my hair out getting over, I use an ethernet card instead of the motherboard's.

So, that's the basic state of things. My problem: what is causing these reboots? Why can't my system be stable or handle the kind of loads an AMD Athlon XP should handle without problem? My system temp seems to be okay- when it shut down last, the temp was 107 deg F. It usually runs around 98 F on start-up. I tried lowering the FSB mhz speed, but it won't start up properly unless I have it at 166 mhz (erm, the setting that lets it run at 333 mhz... I forget the terms, but my motherboard has built in settings to let me overclock (which I DO NOT DO) and adjust settings). The heatsink and fan I have SHOULD be more than enough to keep this baby cool.

Goal- I want to find out what is wrong. If I can't fix it, I'll replace whatever needs to be replace. Hard disk file problems? I'll reformat (if I ABSOLUTELY have to), or replace hard disk (what I will probably do if old HD has problems). need new graphics card? Fine. New motherboard? If I have to, I'll save up for it. New case for better air circulation? Sure, no prob. But what I won't do is replace everything randomly until I find out what is wrong.

Any ideas? What info can I give to help you aide me in assessing the problem? This reset issue has gone on since the system was first put together by me- and I had a professional tech look it over and make sure eveything was put together correctly during config one (before I upgraded processor).

Most sincerely,
Patrick