System unstable

[elec999]

I have a Asus P5K and Q6600, 2x1gig ddr2. System is very unstable. It reboots and freezes. In both Linux and Windows XP. I tried different power supplies, different ram, different power supplies. I have the latest bios. I am starting to believe this my motherboard. My temps are good. And this system used to be fine.
Thanks