This may be an odd question, but what about the NIC polling option? Try
turning it off if not using Intel NICS.
I've never had a lockup on any m0n0wall machine.
I have several m0n0walls that have been running for many months without
reboot. Various equipment, but all using Intel NICs. One is a Dell 533
celeron machine, another is a PII-450, etc. All have at least 4 active
NICs. Various loads.
Commonly using NAT, 10-20 Firewall rules, DHCP, DNS forwarding, Captive
portal and traffic shaping (20-30 rules.)
Wireless is handled by external APs.