My iMac (21.5-inch, Late 2012) started to do something strange when it wakes from sleep. The screen comes on for about a second, then goes off again. The next wake is then fine. It does this each and every time I wake it from sleep. I originally thought it might be something to do with f.lux, as I installed it around the time it started happening. However, removing it didn't make a difference. I have tried resetting SMC and NVRAM but that hasn't helped either. I'm running El Capitan if that makes any difference. I'm at a bit of a loss, so any help is appreciated!