Try to Reset Your PRAM to see if that helps.
Plus if the Reset Your PRAM doesn't work it might be worth to try to reset the Airport card. This is drawn out process that you must follow the steps in order:
1. Open system Preferences->Network pane, highlight the Wi-fi airport card and in the right hand pane turn OFF the airport signal. Then quit System Preferences.
2. Navigate to the folder /YourHardDrive/Library/SystemConfiguration/ and find the file: com.apple.airport.preferences.plist and drag to the Trash can (but don't Empty the Trashcan just yet.
3. Immediately Reboot to rebuild that file in the /SystemConfiguration/Preferences/ file.
4. Upon the reboot go back into System Preferences->Network pane and add you wireless card back and rejoin your network. In the right hand pane mark sure you push the 'Apply' button tom save.