XP Client drops off NT4 Network

I have an XP Client on an NT4 Network, SBS 4.5 to be precise.
The client connects to the network but then drops off. This happens erratically.
The client will only connect with a fixed IP address.
If the client machines IP address is changed to dynamic and then changed back to fixed then the client will reconnect to the Network.
The NT Server is only running TCP/IP and NetBEUI.
The XP Client is running TCP/IP, IPX/SPX
It also has QOS installed in networking.
WHY IS THIS HAPPENING and what is the solution. MS are being hopelessly unhelpful
PLEASE HELP