PCI-E slot giving artifacts?

I think I have a junk of computer that is giving me a headache:

Long story short:

I have two computers:

Abit FP-IN9 SLI with E4600, 4GB micron D9 ram, Corsair VX450, and two 7600GT SLI'ed.

Abit ix38 quadgt, E8500, 4GB OCZ fatal1ty, Muskin 650W psu, and sapphire HD4890.

And the problems begins:
The HD4890 run in that configuration ran ok for two days then it gave artifacts in windows and right after even in bios/post screen. (the VGA card is running at 71degrees Celsius max in furmark, so it's not an overheat problem, and i hadn't OC the vga at all)
In the next morning was still giving artifacts right from the start (I mean graphic artifacts not ascii, like red checkerboard). After half an hour without touching anything inside the computer it ran perfectly again.. for 10minutes then it started artifacting.. and the cirque began:

I thought the VGA card is toast, but I runs flawlessly in my second computer (the fatal1ty with e4600 and vx450 psu). And I started thinking the abit ix38 motherboard has a problem.
I've put an 8400gs vga (a junk, I know) on the ix38 quadgt and it runs flawlessly also. (so is 7600GT from the other computer into this one)

So the VGA card (4890) is perfectly fine, and so is the motherboard? then WTF are the artifacts doing??  

I've put back the 4890 back onto ix38 qudgt and it artifacted right form the start, a quick restart and then it worked really good without problem for the rest of the day.. weird. the next day was artifacting again in window, bios, etc. I've done an update of bios of the motherboard, popped in the 4890 and it ran really goot and i sayd to myself "good job". Two days later (today): artifacts during a game, then in post screen after restart. Got all mad and popped the 4890 into the second PCI-E 16X slot and artifacted right away. I put the 4890 into the third (middle) pci-e slot (4x) and ran perfect from 8:00 to 15:00 when I left. I returned at 22:00 and after 10min it was artifacting in windows then in bios, (in the middle slot 4x).
I've done a quick restart and works ok in this moment (in the 4x slot).. still

I have tested the ram with memtest - 0 errors , (and in the meantime I've upgraded the ram from kingmax value 800 to ocz1066

I've reset cmos countless times, I've updated bos, I've updadted drivers, I've tested different ram, I've tested different psu (vx450 runs ok with this board, so as the mushkin 650W), even different operating systems (vista, winXP, knoppix)..

WTF? the VGA board is fine on any other MB, and other VGA cards* work with the ix38 quadgt

*ideea: the MB could still be the problem, but i don't know: the other vga card i've test are really entry level, low tech crap like 8400gs or 7600gt, maybe on high transfer rates on more performance VGA cards the MB have problem, but the protocol is the same as far as I know...

the next thing to do would be to lit it on fire and throw it to the end of the universe.. I had enough of it, but before losing my mind.. does anyone have any thoughtful hints I can follow? (i hadn't update the bios of the vga card.. yet, maybe it helps)



Edit: and I have to add a crucial detail.. the MB has it's problems like it's not showing the temperature of the CPU (it's 34 degrees Celsius at any time, fixed to 34, after 1h of orthos.. 34degrees, in bios also 34 degrees, in any software.. 34degrees) - this is probably due to a broken pin in the lga which I know of, but runs ok, stable, even with OC (4GHz) the performance is there in all tests, no problems from that.. I could repair that LGA775 socket but it's really a big fuss to do that with little success rate