OS X Mavericks Screen brightness auto adjusting even when option is disabled
Solution 1:
I was just struggling with the same issue—it really annoyed the hell out of me. What helped me was a good ol' SMC reset:
See http://support.apple.com/kb/ht3964 on how to do it.
Solution 2:
It may have to do with your charger.
I had the same problem; look at your charger, the lights probably keep changing from green to orange to nothing. When you plug in your adapter it automatically adjusts your brightness, the bad contact makes it do this over and over again.