I Have seen this in my brother's laptop, some forums say it is a display panel issue, others say it is a video-ram issue, try hooking up annother display, if it does the same, your computers graphics card is bad, also, which apple display, is it a Studio Display (3 Legged Picture frame like thing with powermac G4's and early G5's) or is it a Cinima Display (Newer Aluminum Displays Matching The PowerMac G5 and Mac Pro) or is it a LED Cinima Display( newer LED Backlight with a Mini-Displayport connector for MacBooks and Mac Mini's)?