apologies i posted this in iMac forum and got no answers, but maybe its not specifically an iMac problem, I've no idea if its hardware or software related. just in the last week or so my mac has started, maybe only once or twice a day, flashing to a black screen just for half a second. always when I'm doing something, never when its idle. the easiest way i can explain it is either its a bit like when you watch someone logging into your mac using screen sharing (I'm not suggesting its this) or your screen saver goes to kick in. now its an iMac 2011 so I'm scared its the graphics card going, but i don't think so. it could be something software related, i recently downgraded to yosemite from el capital beta. maybe i should try upgrading to the gold master. ill twice a day, sometimes playing start craft 2, sometimes in safari, the screen will go black, its not a flash but its not long, half a second.