VPN Access

I set up one of my Windows 2000 servers as a VPN server using “Routing and remote access” users dial into their ISP and connect to the server fine, they can also ping both interfaces on the multihomed server, but they can’t always get to the rest ofthe network. They’ll be able to ping other servers and workstations and then suddenly the pings will start timing out, it’s very unreliable. Any suggestions are welcome.