My Fujitsu lifebook T902 keeps changing the screen brightness levels for no apparent reason.

This happens most often when I am writing on it in tablet mode, but sometimes in laptop mode. I have adjusted the brightness in the control panel and the power settings, but regardless of what the settings are, or whether it is plugged in or not, it still keeps automatically adjusting the brightness at random times. The only time this doesn't happen is when I have the screen set to the lowest brightness setting, which is not an option most times. I use the tablet mode quite a lot, so the constantly changing screen brightness gets very annoying very fast. Particularly when there isn't any apparent reason it should be doing so.