Same problem over here, after about 24 hour of operations only small IP frames get
Natted outwards to internet all large ethernet frames (>1450 bytes) are
dropped. Reset of router of M0n0wall fixes the problem for 24 hours.

Setup:
-generic 386PC image M0n0wall version 1.1b16 on an Advantech SG-1003 200MHz embedded system
-Dutch KPN ADSL with pptp client to Alcatel speed-touch modem
-Enable advanced outbound NAT
-multiple Inbound port translations from internet to internal network
-no DMZ connected
-no traffic shaping
-I never tested version 1.0 of M0n0wall.

Fix?:
I am currently testing with the Fred Mol interface settings, in de last 24 hours the problem has not
returned yet so it seems to be fixed.

With kind regards,

Paul Boot,
Delft, The Netherlands.