To help stabilize your connection, please try lowering the MTU on your VPN virtual interface, which you can accomplish by issuing the following command (where tap0 is your VPN interface that is connected to the labs):

ifconfig tap0 mtu 1200

Continue lowering the MTU value in increments of 50 until you find that your rdesktop session is working properly. You can also add the -P and -z flags to your rdesktop command to enable local disk caching and compression respectively