VPN sessions and ip_conntrack_max

Hi

I am creating some boxes that are used to provide access to the
internet.

These the computers behind these boxes often establish vpn sessions to
remote sites. when one user drops a connection to a site, and another
attempts to create a connection again there is a problem. it takes
quite a while for the previous session to be dropped before the next
user can create a connection.

Anybody got any ideas how i can get round this.