A few months ago I install Mountain lion in my imac 2009.
Since then I have noticed that when you start the system, the initial screen where the logo appears, I dyed yellow. That does not worry me too much. But once in the system, when I try to play any video in full screen, it generates flashes that make viewing impossible. This problem I have encountered with playing from Quicktime but whne i play in VLC no, for example. The same problem happens to me when I play videos from youtube.
You may have problems with the graphics card imac??? or is a problem of Mountain lion??? Before Snow Leopard-and never had I had encountered this problem.