Problem with thernet card and dhclient

[Stephen Montgomery-Smith]

My computer running a very recent FreeBSD release 5 is connected to the 
internet with "3Com 3c980C Fast Etherlink XL" to a cable modem connected 
to Mediacom.  I connect with the command "dhclient xl0".

The problem I have is that every so often the mediacom connection goes 
down, and when it comes up again, the computer seems unable to connect 
with the outside world.  Yet if I do "killall dhclient;dhclient xl0" it 
happily connects sufficiently well to get itself an IP address from the 
mediacom dhcp server, but it does not seem able to pass any other kind 
of internet info, e.g. ping, nameserver, ssh, etc.

However rebooting the computer completely solves the problem.

It is as if the IP software gets stuck in some weird mode, and seems 
unable to get out of it.

Anyone got any ideas?  I'll provide more details about the system if 
that will help.

Thanks, Stephen