Server stops accepting connections from WAN but works fine on LAN

We recently deployed a new CF MX8 server on Windows Server 2003.  About every couple days I get a call stating that our website is down. I would test from an outside connection and in fact it would be down.  But if you browsed to the sites LAN IP it works fine.  A reboot of the server cures the problem for a couple of days. Any ideas ??