VPN drops connection frequently

We installed Ciscos VPN client 3.6 on both XP Pro and w2000 machines. The VPN connection can be established, but drops after 30 seconds to 30 minutes. We are using DSL and seem to have no other Internet or network problems (win2000 server) I added ForcedKeepAlives to the configuration file for the Cisco client. This didn’t help. The DSL provider says our line is clean. Any ideas for troubleshooting this?