This is exactly the same issue I'm having. My setup is very similar:
192.168.0.1 - Router (DNS, Internet)
192.168.0.2 - ntserver (DHCP, File Share)
192.168.0.5 - VPN Server (Windows 2000 Box)
I've port forwarded VPN through router to VPN Server.
I can connect via VPN from the Internet and can log onto the network. I get an IP address, for example 192.168.0.233.
When I ping the Router, it works every time.
When I ping the VPN server, it works every time.
When I ping anything else on the network, either via name or IP address, I only ever get 1 returned ping...The very first one, after that I get nothing from the device until I disconnect the VPN session and reconnect (then the cycle starts again).
Additionally, a client on the Internal side of the network can ping the 192.168.0.233 (VPN Client) address, but the same thing happens, it only ever receives one reply, the first one.
Can anyone help?