Here's a recent suggestion:
1. From Menu bar, Go|Computer|Macintosh HD
3. System Configuration
4. Hold down Ctrl and click on Preferences
5. Get Info
6. In Name & Extension box, add .old to System Configuration file.
7. Shut down system & remove power lead.
8. Hold down power button for 15 seconds.
9. Reconnect power and turn on.
System will create a new SysConfig file without the corruption which may have caused the WiFi problems.