lost connection to windows NT-based server

When I try to connect my Windows 98-based computer(Window
98 second edition) to a Microsoft Windows NT-based server,
it will lose connection about 30 minutes after it is
started. If I tried to ping this server by machine name,
there was no problem. DNS and Network worked OK. If I
checked the mapping drive for this server, the icon
indicated that the link worked.But if I right-clicked it
and open property, it showed that status was disconnected.
If I tried to find this computer by either machine name or
IP address, I failed.If I restarted computer, everything
would be OK.

Any suggestion.