It could be the MTU issue. these case study may help,
VPN connection is disconnected after serveral minutes VPN connection is disconnected after several minutes. We have been seeing more and more cases like this one. We don’t really know the causes, ... www.chicagotech.net/VPN/vpn3minutes.htm
VPN Issues VPN connection appears with a red X VPN connection is disconnected after several minutes Missing WAN Miniport L2TP and PPTP VPN XP Client Disconnects After ... www.chicagotech.net/vpn.htm
Bob Lin, MS-MVP, MCSE & CNE
Networking, Internet, Routing, VPN Troubleshooting on http://www.ChicagoTech.net
How to Setup Windows, Network, VPN & Remote Access on http://www.HowToNetworking.com
Why VPN disconnects every 5 minutes when we have NAT Server enabled on LAN
