Why does my macbook pro displays random garbled blocks?
Occasionally my macbook pro (15-inch, Early 2011 with mountain lion 10.8.2) displays random garbled blocks. This happens on external Apple displays and the laptop display. See photo.
Solution 1:
We had a lot of these problems in our store and in the most cases the graphic card was broken. You should run the Apple Hardware Test / Apple Diagnostics and look at the results.