The problem you describe sound like you are losing your master

When you use windows (9x,2K,XP,2003) in a workgroup (ie. no PDC) you
need a master browser.

The master browser is 'elected' at startup based on what O/S's are
already on the network. The role of the MB is to browse for and
remember NetBIOS resources (shared folders, printers etc. using the
SMB protocol) everyone else will consult the MB before going looking
themselves. Once a MB has been elected it will remain until another
election is called.

MB elections are weighted as follows:
PDCs win!
BDCs beat workstations
WinXP/2K beats NT4, NT4 beats NT3.51
NT beats 9x, 9x beats 3.1/3.11/DOS

Linux can beat anyone when using samba; in smb.conf there is a GLOBAL
parameter 'os level = nnn' where nnn is a number (0 to 255), the
higher the number the higher the ranking.

What you describe could be explained by the master browser no longer
being available (ie. someone downs their workstation), the others
therefore have nowhere to look for their browse lists and so the
resources seem to disappear.