Chosen Solution
It doesn’t do it immediately after being turned on, it happens after a while of use. There’s no unusual sound like a fan spinning hard and it doesn’t seem unusually hot. I’ve removed the display and re-seated it and made sure all the connects are firmly in place on the motherboard. I’ve uploaded a short video here: http://mazz0.com/images/IMG_0673.m4v And here’s a still image:
http://mazz0.com/images/mac_screen.png
Your GPU has failed. Here’s how to replace it: iMac Intel 21.5" EMC 2389 GPU Card Replacement
