Mixed OS LAN (WINDOWS) - worked for 4 years, started to go down often

Got a network of 10 computers. Most run WIN 98, some run Windows 2000
PRO. We are using WIN 2000 SERVER - system worked fine for 4 years.
Recently connectivity drops frequently, some machines on network
cannot connect to Internet, yet they are in fact connected to network
and are sharing files. No worms/viruses detected. When PINGing
server connectivity resumes 50% of the time. When browsing through
Network Neighborhood, sometimes it takes a couple of seconds,
sometimes you wait for a minute. No IP addresses are duplicated.

Thank you.