If your Mini DisplayPort enabled MacBook Air, MacBook, or MacBook Pro is exhibiting this symptom and you are running Mac OS X v10.5.6, please perform a Software Update through the Apple Menu. The latest MacOSX Software Update includes an NVIDIA Graphics Update to address this issue. To perform an Apple MacOSX Software Update, please follow the instructions below:
Choose System Preferences from the Apple Menu.
Choose Software Update from the View menu.
Click Update Now.
Select the items you want to install, then click Install.
Enter an Admin user name and password.
After the update is complete, restart the computer.