ip troubles

I've got a linksys WRT54G wireless router, and a SB5100 cable modem. I'm trying to change my external ip adderss. RoadRunner uses DHCP, so I've basically gotta 'trick' it into changing my ip for me.
I used to be able to go into the setup at 192.168.1.1 and set a static ip, but that doesn't even work anymore for some reason. I've tried maybe a hundred different ip's by changing the last two sections. It won't connect to anything for 99 out of 100, and when it does, it doesn't work for anything but the firefox I used to change the IP.
Anyway, the people at RoadRunner keep saying 'unplug your modem and router, wait 30 seconds, plug in modem, wait for the lights, plug in router.' I did that like ten times and I still have the same IP. I even had a new modem installed a few days ago, and my ip was the same after I got it set up.
I can't think of anything else to do, somebody help. :(

Question: are you still able to connect to the internet via this setup? Is it just that you can't acess the router setup pages?

huh
no, i can access the router setup pages fine, but i can't access the internet at all after i close firefox until i go into the router pages again and set it back to dchp