Although i haven't experienced your problem, since you've tried all the usual stuff. what you might concider.
1. use carbon copy cloner, and clone your main drive to an external drive
2. try and boot off of the external drive, if that goes well
3. make a clean install of leopard on your main drive and check if the airport behaves
4. if all is well, you could use 'migration assistant' to move over all your stuff
5. if then the problem returns, it's something you've installed. any dubious software with not-so-clear function of any kind
or you're home free
it's quite a lot of work, i agree, but honestly, i can't think of anything else at this point