"most field lockups i've seen have been caused by an overheated CPU, which 
is fixed by a reboot."


After reading this I did some checking and found very high temperatures in 
my units - all of them 1 rack unit, mounted on top of each other in a 
cabinet that houses a lot of other networking gear (the cabinet is 
pressurized for cooling).  I found the most used box to be very hot to the 
touch (mostly around the RAM) - hot enough that while I was spacing them 
apart it hurt my hand while holding it.  So, after putting one rack space 
between them all (3 m0n0wall's in this cabinet), they have been running a 
lot cooler.

Just over 5 days now and no issues.  I will update as the days go on.

Thanks Dinesh for the idea.


Aaron