Make a note of all your settings for Wi-Fi in the preference pane, then the connection from the connection list and recreate it with the same settings. You do this by clicking the plus-sign icon below the connection list, and selecting as the interface in the sheet that opens. Select from the menu, then select your network. Enter the password when prompted and save it in the keychain.
In some cases, the problem has been resolved by repairing permissions on the startup volume.