If you have two routers on a network, both distributing IP addresses, you can be causing the issues. You need to pick one of the routers to do the routing and have the other just bridge the network.
Are you sure it is a router and not a hub (or switch)?
Bridge mode passes IP addresses through to their respective computers. The IP address comes from the main router.
I can't tell exactly how you have your network setup, but here is what I think.
Modem-->Router-->various computers and Airport Express which re-distributes the IP address it got from the router to other computers.
If that is what you have, go into Airport Utility, Internet pane, Internet Connection tab, and set Connection Sharing to Off. This way, your router gets the main IP address from the modem and then sets up an internal network, giving out individual internal IP addresses to each computer. The Airport Express will act like a "bridge" allowing the IP address to get from the router, through the air, and to your wireless computers. Page 36 of
Designing Airport Networks describes what I am talking about.
