Strange computer fails WoW

[Acer_Sheep]

Hey all

Last few months(when I upgraded computer) everytime when I'm playing WoW with some of max graphics details and in some zones computer is auto restarting and sometimes has been completely turned off after crash. I tested computer and it is 100% stable, no overheating, no overclocking. I have all latest drivers and updates. And every other games runing fine(such as Crysis or Half-Life 2 have no issues, 3D mark Vantage also with no crash). Can't check any error logs/report's because they weren't created after crash and it happen too after I disabled auto restart function.
First I thought it is low performance of PSU which is 420W Silent Giant LC6420GP. I tried other 2 PSUs(420 nad 500W) which I lent from mates but after I installed them into computer nothing happen(only fans and power-on LED were active but no boot, no signal in monitor).
Before, I ofc reinstalled system and drivers. Called game technical support but no fix has made. I'm nearly out of ideas how to fix this.

If anyone have any new idea, will be welcome. Thanks alot

Athlon X2 6000, 2GB RAM(Kingston Hyper-X), GA-MA-790X-DS4, Geforce 8800GT(256 MB RAM), Acer AL1912, Windows Vista 32(tried 64 too and doing the same)