DHCP Errors

I am running Windows NT server 4.0, SP6a, and Proxy Server 2.0. I have a cable modem which is shared thru the Proxy server. It has worked fine for abot 1 year. For the last 2 weeks or so, I get a message stating that ” The NIC could not renew a lease”, so I re-boot the server and every thing is fine for about an hour or so and then the message again pops up. I have moved the cable modem to a 98 workstation and had no trouble keeping my IP address. I also have tried several different NIC’s in the Server, all producing the same error. And lastly, I tried uninstalling TCP/IP and re-installing it and DHCP but that has not helped either. Any ideas? PLEASE!!!!!!!!!!!