I can provoke the black screen by watching youtube videos on full screen but otherwise I have no problems so long as I limit the brightness to 6 notches or less.
I did take the iMac apart some time ago to clean out any dust and have a look at connections and noticed that the monitor cable where...
...several hours later, max brightness, no fans and so far no problem using an apple magic mouse and no usb peripherals. If you don't hear back from me you can assume that my problem is solved. I hope this works for someone else too.
Just joined to share my experience.
I bought a used 27 inch late 2009 iMac about a month ago. At first I was delighted with it. I went on holiday shortly after taking delivery of it and only when I returned and it was too late to return it a big problem showed up. The screen would go black...
