WINS cutoff from Win2000 server

I have a Win2000 Server on a network on a frame relay. In order to connect Win95 computers through a router over the frame to a program on the server, I have set up WINS on them. They work fine for a day, but every night they loose the connection and the pc’s have to be rebooted in order to find the directory on the Win2000 server. Can someone tell me how to avoid the loss of connection?