VPN Works 100% then fails permanently

Hi All, here is the situation:

ATT/SBC DSL: Static IP
Firebox Edge Firewall
Windows 2003 SB Server R2

I have setup VPN with all the correct settings, opened up all of the ports in the firewall and have gotten the VPN connection to work 100% from the “outside”. Then, all of a sudden, after disconnecting from the VPN server, no “outside” clients are able to connect.

Internaly, the VPN still works fine. Externally, however, it gets passed verifying credentials, then loses it on negotiating.

Am I dealing with an ISP issue? It’s not the firewall (the VPN connection working proves this). Is it some setting I have on the VPN Server?

Btw, I am using only 1 Nic, if that makes a difference for anyone.

Please help!!! =)