Symptom
Flashing vertical lines or short, white, flashing horizontal lines appear on a display connected to the computer.
This might happen if the computer's built-in display is closed, and you're playing some types of games on an external display.
Solution
Download and install Mac OS X 10.2.8 or later. It is available in Software Update preferences, or from Apple Downloads (
http://www.apple.com/swupdates/).