Weird System Crashes!!?

I have an older ASUS K8N board with an AMD Athlon 64 3400+ CPU. My video card is an EVGA 7600GT 512MB AGP series. This is what I mean about these weird crashes my system will do. Every three or four days my system will crash upon boot-up but in between those days I can shut it down multi times an it won't crash. I have ran MemTest on the ram and no errors there. Also ran a program to test my hard drive for any signs of failure. There were no know issues that came up. So I had my power unit checked on a volt meter and it was fine. I had the Video card tested on another AGP system and still no errors. I also had the CPU tested on another Socket 754 board and it showed no problems either. Also the board show no errors either. Since my hard drive is a SATA drive we tried different SATA cabels to. So can anybody explain to me what in the HELL is going on with my PC. Any an all hel would be gratefull to me. Oh by the way this issue I mentioned started a little over a weeek ago.