Server has one NIC yet Multihomed errors

We recently replaced the network card an an old server which was wiped (of Small Business Server) and loaded with Windows 2000 and Winfax, to serve as a fax server.

Every few days, this server drops off the network, although it continues to receive faxes - but no longer communicates with the PCs. Through Explore you can no longer access the server when this happens.

The error messages are Event Codes 8021 and other times, 8032 - Multihomed Master Browser may cause these event codes. However, there is only one network card in the server.

Why would it still "behave" as though it had two cards, or, is something else going on here? Your insights are appreciated.