I have had this same problem for over a year now, but it's getting more frequent. I have an 2011 iMac, 27". How do you get to the heat sink for the video card???
For anyone else reading this and having the same problem, although everyone's answer for everything is often reformat your hard drive and reinstall the OS, just and FYI - I have done that twice since this problem started and it does nothing.
Thanks!