Completely random restarts (windows + linux)

Question

Hey,
I put together a system (see sig) for a friend, anyway they started complaining about random restarts using winxp (with or without either service pack). Cutting a long story short, we RMA'd the motherboard, got a replacement, and the problem persisted.
Basically this is a list of things that has been observed to happen
1) Randomly reboots during winxp install (not connected to the internet, harddrive virus free did a complete scan and alternated b/n linux / windows installs, ie filesystem changed -> no virus)
2) Reboots under windows randomly, can be when surfing the net, ripping cds (this is the best test to make it happen), or just sitting idle (went to get a drink and it had done it). I tried to disable the "Automatic restart on fatal error" in the advanced settings thing, that did nothing, so im fairly certain that its a hardware issue after that.
3) Reboots during Gentoo Install (I figured I should stress test it), the reboots occur at different places depending on the settings changed, and detailed below, but inevitably it always reboots.
4) Occasionally winxp will fail to load, or givea a BSOD before rebooting, but this has happened maybe 5 times in hundreds of random restarts, the error messages are not in reference to anything in particular, nor could I garuntee the ability to get a copy of one to show you.
I spent most of yesterday digging around these forums reading suggestions etc...
The following is a list (i took notes) of what I have tried (this is long I know, im sorry)
1) Different PSU
Originally the PSU was the stock standard that came with the sonata http://www.antec.com/specs/true380_spe.html which at 380W seemed a little low (this was before reading forum posts), so replaced it with the one in the sig -- Not a PSU problem.
2) Different RAM
Figured since a second replacement board had the same problem it could be the RAM, tried RAM from my machine which I know is working perfectly, reboot as normal. Hmm so I did a memtest86 scan on the memory left it running 48hrs, that lets memtest86 loop about 7 times (for anyone interested) there was not a single error. -- Not a RAM defect.
Started reading forums here
3) Different Harddrive / Optical Drive
Tried a 120G sata maxtor drive (which I know works), reboots persisted.
Tried a DVD-ROM, from my machine, reboots persisted.
4) Measured the voltage with multimeter, all voltages were within 1% of the spec.
5) Put some thermal (arctic silver 5) compound on the cpu / heatsink, but the temperature under stress test never goes above 51 degrees anyway. Not at all out side the operational conditions for a P4 3.0.
6) Ram Timings / placement
If I leave as SPD, reboots occur more quickly, setting timings to 2.5-3-3-7 burst 8 gives me longer until the reboot occurs, however it still eventually occurs. I have tried running the ram in single channel mode (with both sticks in the same channel), dual channel mode 1 3 and 2 4 have been tested, and by only having one stick in at a time (
7) Ram voltage
I kept alternating b/n SPD and the above timings as I moved the RAM voltage 2.60 - 2.65 - 2.70 - 2.75 (I haven't gone any higher yet as thats the red zone, ie 2.80), the best times I experienced for a gentoo install were with 2.70 / 2.75 however after 20 to 25 minutes of compiling the machine restarts. I couldnt get the machine to restart ripping cds on this setting, but thats probably because I lost interest in ripping over and over again after the 5th time, I did get to 10th rip once, only to have it restart.
8) Clock Frequencies
Tried setting FSB to 201 (as some threads recommend), didnt help. Tried setting RAM speed at 266, 333, 400, and auto, the lower the speed the longer until a reboot, but only a minute or two is gained by 266 over 400. All this is done on the NORMAL (meaning SLOW on older boards) setting.
10) Video card
Originally there was just some old PCI card I had laying around in the machine (lack of money), however now (see sig), either way reboots occured, the video card was my 3rd suspect, motherboard, ram, video, psu, .... etc...
Thats it.
Now, I saw some people commenting (when I was reading the threads) that they wondered if this batch of boards was bodgey, no one seemed to want to agree, however after trying suggestions from over 100 threads (do a search for random reboot if you dont believe me) and not actually getting any success, and having had this happen over TWO of the SAME board, im really beginning to think that is the case. Some of the search results were PSU based, granted, so lets knock off 50% that leaves about 50 cases of this incident which haven't actually been resolved.
Im not playing blame game, it just seems very likely to me, but if someone gives me a solution that works, ill appologise profusely, and go and buy the MSI K8N Neo2 Platinum that ive been wanting -- but to scared from this situation, to actually buy.
This problem is remarkably similar to Random PC reboot this, except im not able to actually produce the windows errors as noted at the top.
So... Please HELP !!!!!

Answer 1

1st of all, kudos to you mate, now this is the proper way to ask for help!  
but looks like you've tried everything other than hitting it with great force with a piece of 4 by2.
only thing i can ask is if you've tried increasing the vAGP in bios, this doesnt really increase the voltage to your agp (well, it does) but more to the point, it increases the voltage to the motherboard chipset. you've mentioned that youve put as5 on the cpu, it might be worth trying a clean and application under the northbridge too (though they do coat it with some cr@p so much its a bit of work sometimes getting it all off)..
another thing, though this shouldnt help, is to try increasing the vCore to the cpu just a tad, most intel boards' voltages vary a fair bit, some say its due to a special functionalities withing the processor itself, but most say that this hasnt been implemented.
other than that, best of luck to ya
p.s. or you could go get a cheap 865 board as a replacement, i wouldnt upgrade to a64 yet with nforce4 (nvidia) + xpress200 (ati) mobos just around the corner as well as 90nm a64 processors with sse3 instuctions too