Airport connections problem

Hi,
since i have installed snow leopard on my macbook i have found sick problems on airport connection.

When i connect it on an router for the 1 time, everything works fine, but when i connect to another router and then came back to the first, the airport connects but cant get an valid IP.

I already tried any solutions on my mac and router but the one thing that works is to restart the router BUT i really don't believe thats the problem is on it because its happens on others routers that i usually connect and before i formatted my mac and installed the new OS 10.6, airport was good. Another reason is that i have other macbook here OS 10.5 that works always fine!

Looking for help!

Thanks everyone!

Have you double checked to make sure that the Network settings on your 10.6 Mac are exactly the same as the settings on your 10.5 Mac. If the settings are the same, both Macs should connect (or not) the same.

yeah, both configurations are exact the same.