TCP/IP & DNS issues

This is a very interesting problem. We have set up numerous windows 2000-workstation on our network. They seem to work fine but after a few days or so, the TCP/IP address seems to become unbound. What I am trying to say is in the morning a user willbe working fine and later on that day or just a few minutes later they can’t brows the web or send mail. You receive you can find DNS error while browsing the web. However, if you do a release and renew that seems to fix the problem but more often or not the user has the reboot. This is become a real nuisance.
We are running Novell 5.0 ad our DHCP servers with Novell client 4.70 and our DNS server are UNIX computers, and I have not found a solution yet.