2k3 Server lockup w/o any information

A 2k3 server locks up. Completely, read again Completely. No response from the machine (video, keyboard, mouse). I have to use the reset button. After the reboot it is fine for another 12 to 24+ hours. The system does not log anything wrong, except for the unexpected reboot. There is no warning, it happens suddenly. Heat is not an issue. I can’t find what is going on. Should I log performance counters? which ones?