VPN - cannot ping internal IP after a couple of minutes.

I have a strange problem.

I have 25 WinXP Pro Clients (static IP's) over 2MB SDSL connection running through an XPPro gateway (ICS and Zonealarm PRO).

I can connect all machines to my Win2000Pro VPN server over the internet and can initially ping the internal IP of the VPN (192.168.41.1).

However they will sporadically drop this connection - the VPN connection is still live and I can ping the IP address of the local PPP adapter but 192.168.41.1 receives "request timed out" although routing in the correct way. Disconnecting and reconnecting to the VPN resolves the issue but is not something I want my users to do several times an hour.

It also happens if I connect a Win98 client, or put a client on a static address in the DMZ.

Help!!!