Have you recently added a couple more IP's to your firewall's external adapter? If this was an ISA server I would say you need to configure your external adapter to set up individual listeners per IP address.

I say this because almost everyone on broadband is going to have sometype of router/firewall in between them and the internet so if they send a request out and one IP on your firewall recieves it, it may reply back on a different IP address. Your router/firewall on your side sees this as someone else accessing your network and denies it access.

Dialup does not have this problem because most dialup connections are not protected.