intermittent NAT problem

OK, I'm sharing internet via NAT on a W2K server.Everythings
working great when suddenly connection is r-e-a-l slow from any client
(98/2K) or from the server. I do a release/renew on the server, everythings
fine again for half a day or so, thenproblem re-occurs. I'm getting a more
or less permanent IP address from my ISP,and it had not changed when the
problem first surfaced (DSL connection).It works OK but it's a hassle going
to the command prompt once or twice a day. I'm new at this so don't worry
too much about stating the obvious.
Thanks,
Pete