First of all cheers 2 all :)

This is officially my first <help me> post, I stumbled on this web after googling the magic words "girls gone wild" and hitting the Im Feeling lucky button, then....oh wait... No that was before. I mean googled "motherboard random shutdown" :$
That said,I read a few topics related to this but none quite solved this little thing I have in hands so I thought perhaps I'd fall in ur good graces :) That and I was happy to find a place where it seemed I could learn some thing from and hopefully leave some handy input.

Well I guess this could have gone into my intro post but it just felt wrong 2 spread my lil problem outta the blue, so here it goes :

Current situation - I cant work (or play) on anything that requires graphic activity
Previous situation (for a month or so) - I've had days, weeks even with no problems at all, on occasion my computer would immediately shutdown and after reboot all was fine, also sometimes I had it running (any 3d app) for a few hours when suddenly computer shutdown. Rarely (cant say for sure,maybe 2 or 3), I've had days where I couldnt do anything.

Trying to fix this situation I have so far, changed my graphic card, tried ram slots one by one, formated hd twice, tried psu on a friends similar rig, updated Bios and chipset drivers.
I've runned ram, hd and processor tests (all in DOS, things like memtest,Seagate hd tools and a few others). I tested it after a fresh reinstall with only necessary drivers and Ati tools and I've "managed" to see it shutdown again. Whats annoys me more bout this is the random factor, only thing not random is the graphic part. Graphic is new, so I'll exclude that. The hardware tests I've run so far give me no reason 2 worry about any piece but Im looking forward for opinions.

A few things that might matter:

At certain point I had 2 change my net connection from lan to usb, for some reason in a lan meant instant shutdown only (again annoying) in 3d applications. Worked fine for a while, I actually thought I had figured it out, some driver problem of some sort. Well its back

I had once a checkdisk after a shutdown, for what I understand it might be due to bad sectors. Never happened again thou.

I had today a bsod prior to shutdown, it said a process had been terminated causing a fatal error.

My specs:
Athlon64 3,5, 3GB Ram, 2 120GB HD, Asus A8N-E MB, HD 2600XT, inboard soundcard, 500W PSU, CADSL4 Router (I' actually read about modem/graphic cards incompatibility, is it possible?)
Windows XP Pro SP2

Idle Temps: Board: 30~33 CPU: 30~33 Graphic Card: 44
3d app Temps: Board: 33~38 CPU: 40~42 Graphic Card: Cant say for sure (and cant test) but I remember it going to 65ºC in Atil Tools and holding. (also most games I tried dont stress it as much as Ati tools, never checked temps on 3d max :s)

If any1 is familiar with Atil tools perhaps they now whats the "temperature of temp. chip"? I have that one at 99ºC on anything graphic but I think I had it 2 when shutdowns were rare.

Aff big post,I hope some of you guys can share some thoughts on this.
Also if u made it this far, ty for your time

Didnt find a edit buttom
On another try I found unable to remain in DOS, I was gonna run memtest again but it shutdown before it even starts.
Next I was gonna try different program, a ram/hd tester and agan it shutdown on the menu.

Im clueless lol

Some observations that may help.

Random reboots can be difficult to diagnose. The same basic fault can produce different sysmptoms.

Video card overheating usually results in a frozen display, not a reboot. An overtemperature CPU should cause a full power off, not a reboot. Your temps appear normal.

Random reboots not accompanied by a BSOD are usually caused by hardware rather than software (drivers).

Since you have already tested memory (assume you ran memtest 86+ for at least 5 passes), the next most probable cause based on my experience is a faulty capacitor either on motherboard or in power supply. A faulty cap may not have any external signs of damage so don't rely on swelling or leakage to determine whether that's the problem. If any are swollen or leaking -- that's a sure sign of faulty caps.

A 500 watt power supply is sufficient for your system provided it is a quality one. Doesn't mean that it is not the cause, just that wattage is sufficient. Connecting to your friend's PC for a short period of time does not rule out a faulty PSU.

If it is a bad cap, reboot frequency should increase over time until they occur within a few minutes or less after bootup.

First thing I'd recommend is swapping power supplies.

Well I was about 2 go out and get me a new PSU when I thought about turning the pc on in a different place. I got a lotta stuff wired here so Im using 2 pieces with 7 plugs each. Computer was in one of them, I moved to one in the wall, alone.

Also I went inside the case to check the psu and board for any leak, wich I didnt find. Took the chance to clean some dust as well. my surprise its working fine for over 5h.
The funny thing is I switched the power plug to back 2 where it was to be sure it was that (expecting computer 2 shutdown) but.. it still works.
I can only assume this is something with the wires inside or some electric charge, maybe the power cable wasnt well attached. The dust wasnt enough to cause overheating,plus I stressed the cpu and checking temps they were just fine.

Im not gonna mark it as solved cause Im not yet sure I can say its fixed, I'll give it some days

Give it several days. Weak caps can cause voltage drops after hours, minutes, or seconds.

If it's OK after a couple of days, then consider it cured.