Earlier this year, the four year old 24″ iMac in my office starting getting horizontal lines on the display. It started with one line near the bottom of the screen, worsened with another line appearing and then expanded to the entire screen having problems with wide horizontal bands.
Google searches led me to believe it was the GPU (Graphics Processing Unit) overheating due to the internal fans not running fast enough. Apparently Apple had chosen to set the fans speeds lower to make the unit run quieter at the risk of damaging hardware due to overheating. Continue reading