I have the same problem on a late 2012 27” iMac after a SSD install. On this one, after the screen is blurry like that, it switches resolutions down to 720p. Then that is the only resolution it will display. The problem is a faulty display connection. It could either be the display data cable or the socket the cable plugs into. The socket itself is easy to break tiny (not easily observable with naked eye) pieces off and get them stuck between pins, meaning the cable will not fully seat or will not make a connection on all pins. I could only see it with camera zoom on my phone. Broke off a corner Piece of metal in connector Anyway, try replacing the $20 display data cable. Or try getting a magnifier to look at the connector on the board. It wouldn't seat at all until I got the piece out. Now it does the behavior I described at the beginning, including the blurry screen. It seems to realize that the display is going wrong though and drops to 720p. So subsequent boots are no longer blurry like that but really...