Just some general suggestions; hope they can be of help.
The first thing I would look at is the VPN connection. Is it stable? If it has a log file you can check for errors there.
If the VPN seems to be fine you can try monitoring the network from your BSD box with
tcpdump |grep "XX.XX.XX.XX" > /home/myuser/mylogfile.log
where XX.XX.XX.XX is your office IP address. It will output a lot of info, so you may want to filter the output further, depending on the output you get.
One possible cause I can think of is that your vpn refreshes its connection every few minutes, and while doing so breaking your rdesktop connection.