LAN browsing problem

I used to use Windows Explorer from any Workstation in the Domain to browse entire Network. I should see all Client PC on my domain (under “My Network – Entire Network – Microsoft Windows Network – . Recently, I often could not browsed to my domain. But I still map the share using URC path, I could ping them… The big problem is it is not stable, sometime I could and sometime I could’n.

I have only one DC running Windows 2000 Server and 20 Workstation (W2K pro, win98, NT4.0) all have the same problem.
Do you have any idea?