I had a similar problem and I actually posted it here a while back. I 
was using m0n0 in a VMware virtual machine for my home network with 3 
interfaces (LAN,WAN and DMZ). It kept freezing or rebooting on me few 
times a day. At first I thought it was the hardware, but after extensive 
testing I excluded that.

I then switched for a while to pfSense using the same hardware, same VM 
and same configuration. That worked flawlessly from the first boot, so I 
can only conclude that 1.22 is not very VM friendly.

I'm now back to m0n0 1.3b2 on a dedicated hardware and after struggling 
a bit with my 3com nics everything works. From my experience I would 
suggest you to either switch to pfSense + VMware or use m0n0 on a 
dedicate hardware.

Max