I’ve had trouble with random lockups/freezes on my nForce4 based Asus A8N-SLI Premium mainboard since the beginning. I first thought it was related to my Asus 6600 graphics card – using the open-source nv driver instead of the proprietary Nvidia driver on Linux made things better, but didn’t solve it completely. The system simply stopped responding once in a while and a hard reset was necessary.
A month ago I changed the HyperTransport frequency from 1 GHz to 800 MHz in the BIOS and everything works smoothly since. So if you experience similar problems – give it a try.