We have a Win 2003 Server installed as the DNS server on our office network. It is set to forward "All other domains" queries to external DNS servers (in our case one from our ISP and the alternates from OpenDNS). Our internet connection comes in through a router, which handles the DHCP routing.
Periodically the server simply stops forwarding DNS queries, which prevents all users on the network from resolving internet addresses outside of our local domain. This occurs sometimes just once a day but often 2-3 times a day. Stopping & restarting the DNS Service handles the problem immediately...until the next time.
There are no warnings or error messages in any of the event logs.