During past month one of my Debian Squeeze (Linux 2.6.32-bpo.5-amd64) machines did lock up twice, hard. No response to ARP, dark console, Caps Lock, Num Lock not working, Magic SysRq ineffective. Changing the kernel to 3.2.0-0.bpo.2-amd64 from backports didn't help either.
Temperature and load monitoring doesn't show any spikes before crash.
How should I diagnose and debug such problem?
Is netconsole my only bet?
EDIT: I've already disabled screen blanking:
#/etc/console-tools/config BLANK_TIME=0 POWERDOWN_TIME=0
setterm -blank 0
on physical console.
This time it locked, the screen was still showing login prompt. Since last problems I've run a 6h load test with BOINC (Prime 95) test without any problem.