W2K3 Server loses internet connection every 30 minutes

I'm running Server 2003 w/ Exchange 2003 with RRAS as my
gateway between the local network and the Internet (via
cable modem w/ dynamic IP).

Roughly every 30 minutes, traffic between the server and
internet stops. IPConfig still shows that I have an IP
on the WAN connection, but doing a repair on the
connection (or release/renew) restores the connection
(with the same IP), but the problem keeps occuring. When
this occurs, if I check ARP, I see an outside IP address
listed under my local network adapter (usually my Inet
DNS server) listed as 'invalid' - not sure if this is
related.

This is a fresh install of Server - I had it running
since Feb with the prior install and never experienced
this problem before. I did notice that RRAS did not
install properly when I set it up, and
uninstalled/reinstalled the service. My next step is to
just start over to clean up any other bad installs I did
not catch, but wanted to see if there are no better
suggestions.

Thanks.