VPN(PPTP) with MN-700

I am able to successfully VPN (PPTP) to my corporate network, however, my VPN
connection is disconnected after about 40 seconds (this is very consistently
dropped at 35-45 seconds). Everything else works great -- I can still have
Internet connectivity, etc.

If I remove the router (MN-700) from the system and connect my MacBook Pro
directly to the DSL modem, I don't have this problem, so I'm sure it has
something to do with the router. Does the MN-700 expect some sort of
Windows-only signal from the client computer to tell it to keep VPN
connections alive?

Any other ideas?