Automatic transmission changes gear at random by it self

My car shifts by it self from D to 2 or 3 as I drive. If I select S, then it stays there and behavior is normal, it does not change gears by it self. We tired changing gearbox oil, but no change. The car is mostly in my garage, as I work abroad. Also, the small display at the gear changing handle shows wrong gear number - when in D, it shows 2 or 3 and it is not the same as the dashboard display shows. When I shift to S, the small green lights may or may not show S, but some other, at random and changing. Car is in Serbia, Europe.