Is anyone familiar with the phenomenon whereby the screen of an iMac 27" is often dotted with hundreds of colorful little squares, in certain patterns. They come and go, and they slow down my computer significantly. I have been to an apple dealer and they checked up my computer but could not find anything. The graphic card and the HDD were working OK, I was told. I attach three photo's that I had to take with my iPhone : each time I tried to make a print screen (shift command 4), the squares disappeared and a soon as I had clicked the print screen, they came back ... Very annoying indeed.