Certain conditions can cause your m0n0wall to freeze solid periodically. The amount of time between freezes typically varies, and can be anywhere from a few hours to a few days.
The first thing to check is whether you have any shared IRQ's. This seems to be the most common cause. If you have recently rebooted your m0n0wall, you should be able to see the boot messages under Diagnostics -> Logs, on the System tab. Otherwise you can go to /exec.php on your m0n0wall and run 'dmesg'. Look through the boot messages and make note of everything you see being shown with an IRQ. This includes your NIC's as well as other devices like serial and parallel ports, etc. An example of some dmesg output follows.
sis0: <NatSemi DP83815 10/100BaseTX> port 0xe000-0xe0ff mem 0xa0001000-0xa0001fff irq 11 at device 18.0 on pci0 sis1: <NatSemi DP83815 10/100BaseTX> port 0xe100-0xe1ff mem 0xa0002000-0xa0002fff irq 5 at device 19.0 on pci0 sis2: <NatSemi DP83815 10/100BaseTX> port 0xe200-0xe2ff mem 0xa0003000-0xa0003fff irq 9 at device 20.0 on pci0
The above example shows three NIC's with IRQ's 11, 5, and 9.
If you note any two devices using a single IRQ, you may need to try other PCI slots, if possible, remove unused cards (like sound cards), and disable unused devices in the BIOS (serial ports, parallel ports, etc.).
You might want to try resetting your BIOS configuration to factory defaults, and then disabling any Plug and Play OS settings. Also check that your BIOS is updated to the latest revision.
Use hardware diagnostic utilities to ensure your RAM and system in general are functioning properly. The Ultimate Boot CD has several utilities for testing CPU and memory.
Hardware overheating is another common cause. This issue has been noted on WRAP hardware especially when using miniPCI cards. It's also possible and has happened with any type of hardware.
If nothing else, it may just be hardware or a combination of hardware that doesn't play nicely with FreeBSD. You may want to try different NIC's or a different system. This especially seems to be a problem with some old AMD K5 and K6 systems, though some work fine.