Roadjull

Ok. I'm not 100% sure that this is the place to put this thread. If I was sure as to where this belongs I probably wouldn't need help.
The problem started about a month ago. I own a system composed of an Asus A7N8X-D, with a 2600+@2200Mhz, 512 RAM, Leadtek Ti4600 and 2 WD HDD (40, 80). I usually leave my computer running 24/7. Since a month ago I started finding it @ login screen in Win XP once in a while. I didn't think much of it, as I blamed it on power outages (frequent at the time).
But then, about 2 weeks ago, it started reseting while I was working or playing on it. No BSODs, no "system has recovered from a serious error", no anything. It just went to black screen, monitor went in stand-by and after a few seconds it would reset. Hdd was working while it did this.
I am playing a lot of World of Warcraft atm, and that's where it agravated. It would go in black-screen at random intervals, then the monitor would go to stand-by then it would reset (sometimes by itself, sometimes I had to do it) After reset it could either work fine for a while (most of the time when it was left shut for a few minutes) or just go black @ windows logo screen.
A week ago it got worse. It now goes into black-screen 98% of the time @ windows logo. The other 2% it boots ok, reaches desktop and then dies.
First I thought it was a power supply issue. I own a 430 w Antec True Power. So I took out all the ROM's and supplementary cooling fans and gave it a go. Same thing. Then I thought maybe the O/C was dealing damage to temeprature levels. So I reset everything to default in BIOS and opened the case door for better airflow but that didn't work either. Then I thought my Leadtek Ti4600 has finally succumbed, so I took that one out and placed it in my other comp (pentium 4 @ 2,4). To my surprise, it worked just fine, no problems whatsoever. I then took the video board that was on the Pentium 4 system and put it in the Asus MB. The result was that I got artifacts, screen freezes @ windows logo etc
My conclusion was that the MB was the culprit... prolly AGP slot issue of some sort.

And just as I was about to buy a 939 socket board and switch to 64, the pentium 4 system which still had the Ti4600 inside did the same act. Same black screens, same stand-by issues. To top it off it now displayed artifacts. With my stress levels reaching sky-high I borrowed a 6600 GT from a friend. I installed it first on the Asus A7N8X system. Same black screens, same everything. I even got a black screen while changing master-slave settings in BIOS :|
Then I placed it in the pentium 4 system and it works fine over there as we speak. I should mention that I never changed the monitor. Kept on both systems. It's a Sony E430.

Also, both systems are running Win XP Pro, are up to date in regard to drivers and BIOS updates and both are trojan and virus free.

Any ideas as what to do to the Asus system? Please help.