RDC connects then drops connection and cannot connect again w/o reboot

Host system is Win XP Pro behind a Linksys BEFW11S4 with port 3389 forwarded
to static ip address on LAN. Verizon is DSL provider in Pittsburg area.
TZOlite is running on host system. I am connecting with another Win XP Pro
machine. When the host is initially started (rebooted), I can connect with
RDC. However, after a few minutes (5 to 10 minutes), the connection is
dropped. I can not reconnect unless the host computer is rebooted. If the
connection is initiated thru a request for "Remote Assistance", the session
will run without any problem. The BEFW11S4 has the most current firmware and
the Keep Alive is set for 30 seconds. I have tried putting the local port in
the DMZ, but this has had no effect.
All help appreciated.
TIA
Bill