Arp cache needs clearing or machine stops responding

[Ray Seals]

I have a FreeBSD 5.0 box running IPFW in the OPEN configuration mode. 
Using natd to hide my inside addresses.  The Internet connection will
work fine for about 5 minutes and then stop.  When I mean stop, the
internal interface will allow me to ssh to it but when I try to ping
something on the Internet, no response.  The only way to get it to work
again is to clear the arp cache for the default gateway entry. 
Currently I have a cron job setup to clear it every minute.  

Any ideas on why this would happen?
-- 
Ray Seals <rseals at vdsi.net>