I have a mid 2010 27" iMac. I noticed whilst browsing the internet that there are lines of dots moving up my screen so I set the background to a sold colour and took the following video (be sure to watch in 1080p otherwise it's just a blur): http://www.youtube.com/watch?v=VD11NkxCoBI As you can see, the dots move up the screen. There are 4 lines of equally spaced dots, then no lines, then 4 more etc. all the way across the screen. Has anyone ever heard of this problem before?