Q: Why isn't my MacBook "seeing" wi-fi networks?

My Macbook has developed a fault in its ability to pick up wi-fi networks. It was fine for the first couple of years. Now, when I scan for networks, it often registers them only weakly (on the "bars" icon - menu bar) and sometimes not at all. It often times out joining networks because the signal is so weak.  I know the network exists and is "broadcasting" strongly enough because my iPad picks it up and joins it happily. Sometimes re-starting the computer solves this problem, but only temporarily, and this is unreliable. If I sit right next to the router, it is *mostly* OK, but this is hurting my back. Can anyone suggest what's wrong?

MacBook Pro, Mac OS X (10.7.1)