Possessed iMac 27: Brightness control turns itself down while I'm using computer, fights me when I try to turn it back up

This computer is possessed! It literally seems like a ghost is pressing the brightness key on the keyboard while I'm working. It happens sporadically and seemingly without reason. The brightness icon pops up on the screen as the screen quickly dims. When I try to turn the brightness back up, it is like playing tug of war with the ghost. I turn it up, it instantly goes back down. After 20 seconds or so of this foolishness, it stops and I can resume working. Until it happens again. AARRGGHH!!!

Thoughts?