VPN maping

VPN maping
I have setup a vpn from a win98 client to an nt 4.0 network. I am using a cisco firewall on the network setup for vpn, and a cisco vpn client on the Win98 machine in my house. I can setup a tunnel with no problem and keep it up.
Butafter I map a drive from the client I cannot seem to keep the map for long period of time. After about 2 or 3 min. I have to disconnect the map and re connect and continue. But I never loose the tunnel.
I think it is a setting on the NT 4.0 serverbut I do not know where.
help
Roy