IMac Screen Brightens Randomly

My iMac has the screen set to maximum brightness in the system preferences.
In the past few days, I have noticed that the screen occasionally gets brighter. This is a very fast increase in the brightness of the display - definitely enough to be noticable as it distracts me when it happens.
I would says it happens 5-10 times a day now, and it definitely did not used to happen, it's a recent thing. At all times, System Preferences shows maximum brightness chosen for the display.
This is NOT the auto-reduce-screen-brightness feature of Energy Saver. It doesn't happen when I use the computer after a few minutes of inactivity - it happens right when I'm in the middle of typing/surfing whatever.
There is no corresponding screen dimming. It's as if the screen very gradually dims by a small amount over an hour or so, then suddenly returns to max brightness.
Is this a hardware problem? Or could it be affected by video drivers or some other software issue?

I have been having this same issue i have tried many times to turn off the auto adjust brightness but the issue still occurs, this happens when you have your brightness at maximum or in the middle, it just seems to happen and as you said it is very distracting! Someone please help...  This is happening on my 2011 iMac 27"

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