Win98 drop a mapped drive from NT4.0 aft

Simple lan network with one segment and running NT4.0 and nw5.0 servers.
a mapped driver from workstation (win98) to Nt4 server drop everyday after it been up for few hours, all other users are fine but me being the administrator, my boss and one more user. only 3 of us have this drop problem. we can still ping the nt server by IP and by name.
addition note: from network neighborhood we can’t see the server name. we needed to use the netBIOS name in order to map the drive. also only this nt server mapped drive drop but not the netware mapped drives.
The Nt and the application are 3 months old in this 100MbT network, but we always runs IPX\SPX and TCP\IP.