Preserving connections on NT4 server

[Killbat]

In the house here we have SWbell's ADSL service, served up to the four or so computers through a slap-together NT4 server running the usual Microsoft proxy software. Problem is, SWbell has fewer IPs than users nowadays, so their IPs expire after, I believe, 90min, so our server is set to release and renew the DSL modem's IP every 60min. So, whenever the 60min mark rolls around, the IP is released/renewed, and all connections are broken. Downloads, game sessions, etc. are all cut off. Now, SWbell's recent client software handles this somehow without breaking connections. What I want to know is how to configure the proxy to preserve open Internet connections when the modem's IP is reset (it rarely (if ever) changes, but it must be reset). Any ideas?