Wake from sleep problem

[Sebby]

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!

 

[Sebby]

Bump.

 

[Sebby]

Update: Installing the 10.11.5 combo update seems to have sorted the issue.

 

[Sebby]

The issue reappeared and I would guess it was installing f.lux again that did it. I removed f.lux, installed the combo update again, and all is well. Will monitor things without installing f.lux this time.