I used a motherboard that used the forcedeth NIC driver and that caused a lot of hard lockups for me. Changing to a different motherboard solved my problem. I've been using the M2N-SLI Deluxe for quite a few months now with no hard lockups.
Yours is also likely a specific hardware/driver issue
Yep... I think thats good advice.
Our demo Core is regularly up for 8-10 weeks at a stretch without reboot. When it rebooted is only ever because we have upgraded it or changed the config in someway. It is currently running on a an Intel 915 based motherboard.