Firewall / ipFilter Dies

Periodically the firewall on one of my servers just stops. It restarts quite happily when told to do so (11:23 am below, for example), but a firewall that takes naps is no firewall at all.

There are no conspicuous errors in /var/log/ipfw.log:

Sep 2 20:48:28 blue-aspic servermgrd[54]: servermgr_ipfilter:ipfw config:Notice:Flushed IPv4 rules
Sep 2 20:48:37 blue-aspic servermgrd[54]: servermgr_ipfilter:ipfw config:Notice:Flushed IPv6 rules
Sep 3 09:52:37 blue-aspic servermgrd[54]: servermgr_ipfilter:ipfw config:Notice:Flushed IPv4 rules
Sep 3 09:52:38 blue-aspic servermgrd[54]: servermgr_ipfilter:ipfw config:Notice:Flushed IPv6 rules
Sep 3 11:23:38 blue-aspic servermgrd[54]: servermgr_ipfilter:ipfw config:Notice:Enabled firewall

But I do find one curiosity when filtering all logs for "ipfw":

9/3/08 9:51:08 AM com.apple.SystemStarter[51] ipfw: rule 10 does not exist
9/3/08 9:52:37 AM servermgrd[54] servermgr_ipfilter:ipfw config:Notice:Flushed IPv4 rules
9/3/08 9:52:38 AM servermgrd[54] servermgr_ipfilter:ipfw config:Notice:Flushed IPv6 rules
9/3/08 11:23:38 AM servermgrd[54] servermgr_ipfilter:ipfw config:Notice:Enabled firewall

I'd appreciate a hint or two... I'm not even sure where to look next.

Thanks.