I clipped the following from an old article a few years ago (about the time that your computer was new) and can't find the original to give credit......
Quote:

Delete all “Preferred Network” listings then re-establish. One surprisingly effective AirPort Base station-specific fix for an inability to connect to wireless networks involves deleting all stored preferred networks, then re-establishing any used listings. Here’s the process:

1. Open System Preferences and select the “Network” pane
2. Select “AirPort” and click “Configure”
3. In the “By default, join:” pull-down menu, select “Preferred networks”
4. Delete the network(s) you regularly use from the list
5. Launch the “Keychain Access” application located in Applications/Utilities.
6. Click on the “Kind” filter at the top, and look for “AirPort network password” entries. Delete them.
7. Restart, or log out then back in.
8. Repeat steps 1-3, this time re-adding your regularly used AirPort networks to the list using the “+” button.
9. Restart or log out then back in.


You should definitely follow Artie's advice to install the latest updates as well....

Let us know how it goes.

Freedom is never free....thank a Service member today.