Network browsing unavailable

Hi there

We are running W2K SBS with 25 users,NT4WS,W95/98 and W2K workstations. Ocassionaly, when trying to browse network neighborhood, an error message appears: “\\Domain name\” is not accessible.An invalid operation was attempted on an activenetwork connection.
After few hours it is O.K. again. Any idea what may cousing it?