I've been having the same problem for the last couple months even to the current releases of the embedded version with long-term stability. What happens is basically 1day to 1week after being up the router will hard-lock(doesn't ping, webpage non-responsive). When switching over to m0n0wall 1.2 it is perfectly stable, from what I can tell there is something about the atheros wireless card I have which it doesn't like. Anyone else have a similar problem/solution?
Try removing the wireless card and report back if that solves your stability issue. My wrap runs stable like a rock with a cm9 wireless card. also check your powersupply. m0n0 doesn't utilize the card and I think it therefore uses less power. I'm running wrap 1E and 1D hardwarerevisions.
Remote syslog the logs to another host so that you can tell what happens right before it locks.
My 4801 locks up from time to time also. It's got a wireless board in it too. From what I've seen the 4801's ship with supplies barely big enough to handle the unit itself assuming the supply is 100% in spec. I've seen low voltage supplies lock up 4801's with great consistency before. This is probably power related.