Xp reboots

Hi i have an asus m2n nf430 m/b, amd sempron 64 3400 that has started to reboot and gets into a cycle of rebooting. sometimes it only gets to the logon or ocasionally it stays on fo 10 minutes, but if i unplug the network lead it is fine. I have changed the lead, changed the position on the network, suspected the onboard lan, so put in a lan card,instant reboot again. Disable network connections everything is fine all the hardware is only 2 months old. Shop says everything check ok.
Any ideas