MacBook Pro - MBP Airport problems

Hi,

I have a 17" Macbook Pro Core 2 Duo. It is one of the last releases before the new unibody MBPs came out. It has a 250gb HD, 4gb RAM, and a 2.4gHz processor. At my home office, I use an Apple Airport Extreme, but also use various generic wireless routers at other offices and meetings.
Here is my problem: when I run the machine using the built in airport, it cannot maintain a stable connection. It keeps dropping connection and struggling to find a fix. What can be causing the problem? Can airport itself have failed? I had installed the software for both a Vodafone and 3 USB mobile modem. But have done a hard re installation of Mac OS X since it started, without re installing the mobile broadband software and it still has the same problem. When I drop the WAP speed on the router, it seems to solve the problem and the laptop can maintain a stable connection.
Any input is greatly appreciated!

Nigel