Same problem here. There are more possibilities for causing screen brightness variations. I have this contribution: when the battery is around 99% and the mains is connected then watch to the indicator in the lower-right screen edge. When the mains connector is shown for a few seconds (symbol beside battery indicator), on and off, AND the screen brightness is varying in that same rhythm then you have the same problem as I have in two different Lenovo laptops. Explanation: a fully charged battery is not charged further (that is good), but discharged by disconnecting the mains power supply (via software). That is not good for two reasons: life of the battery is reduced (Lenovo should keep the battery in rest) AND when powered by the battery, unwanted, the screen brightness is reduced (depending on your settings for brightness-on-battery). The variation is in the seconds-range. As long as Lenovo doesn't solve this malfunction there is only one solution: set brightness on battery equal to brightness on mains...