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. 

-Kevin