My Toshiba laptop was bought back in 2012. It works perfectly fine until a while ago, when it started to change brightness by itself. I'm sure it was not the power plan because it happens even when I'm using it.
This started happening not too long ago, when I had to use my laptop in the dark and so I adjusted the brightness to the lowest, ever since then, the brightness keeps hopping back to where it was every 2 minute or so.
I can adjust brightness using Fn+F2/F3 and it has a brightness icon with a slider under it to adjust, it disappears when I let go of the Fn and function keys. I somehow managed to get this stuck on the screen so I can observe it, no surprise, the slider hops to the left every now and then.