Airport keeps turning off

I have an early 2008 white macbook and the last year my macbooks airport keeps disconnecting from the internet. It doesn't even tell me that it has. Te airport status in the menu bar shows full signal but i go into system preferences and it says it's not connected. I've tried resetting the smc with no luck. It's getting frustrating as I have to restart my computer to get it to work again and even then sometimes that doesn't work. It's also getting more frequent. I hope you can help me as I can't afford to get it fixed.

thanks

Jamie

I've tried a number of different internet connections and the same in each place.