SBS2003 VPN

I have an inbound VPN set up on my SBS 2003. The
connection from the client computers is irregular. It
will connect after I have configured the connection on
the server but then I am unable to connect after a period
of time (a day or so). I then just reconfigure the
connection on the server with all settings the same and
the connection works and the process is repeated. I
cannot isolate what is causing this connection problem. I
suspect that it may have something to do with the IP
address allocated by the server for the incoming
connection.

Any assistance would be appreciated.