A few days ago I would be sitting in front of the iMac and the brightness would fluctuate instantly 5 or 10 times in about 15 seconds from the 80% I have it set on to about half that and back and forth then stop. It would be fine for awhile then do it again. This happened pre mavericks. I have updates to mavericks, reset the PRAM-SMC etc. I have the autobrighness turned off. Today it has not happened but after turning the computer on after about 15 mins the brightness instantly drops about 20% or so. I checked the brightness keys F1/F2 and nothing has changed. The problem is that it is very intermittent and I doubt the Apple store will believe me. I tried recording it on my iPhone but every time I get to the cam app it stops. My wnty runs out late November. Anyone exp this?