I was on my iMac "27 i7 today and twice the screen turned a dark grey colour, once in windows (bootcamp) and once in OSX. When I put the iMac to sleep by pressing the power button my desktop re-appears as normal before going to sleep. The same issue has also happened but with a white screen. Upon awaking my iMac it runs perfectly for a couple of minutes then the same happens. Anyone know what the problem might be?