Error 18 in dos printers in win98.

I have a network working with Windows 98. All the pcs have novell client installed. The printers are all created with the net use command. When i start the pcs i have 2ports(lpt2/3) captured and working. In the middle of work the connection is lost.And when i try to remove the ports. In fact do anything with net use, they return ” Error 18: (null)”. restarting the pc; returns them to normal; but the error continues: can anyone help me????