At my workplace I had the problem that any outgoing VPN connection got interrupted after 2-4 minutes. We finally found the reason for that was the MTU size that some router was not able to handle without fragmenting. To fix the issue I followed those howto articles:
http://www.howtonetworking.com/VPN/mtu4.htm
http://www.howtonetworking.com/VPN/mtu3.htm