Null Object should not "bounce" when changing direction

Hi,

I animate a picture with an extra mask (as alpha channel) with a null object like this: it moves to the right, stops at the next key frame and immediately moves to the left. But instead making a halt exactly at the stop point it moves on after the key frame for one or two frames and only after that moves in the opposite direction as wanted. This way it creates a small bounce effect that I do not want. 

The only work-around I know is that I add another key frame after the stop with the exact same position and then change direction. But this is annoying on the long term.

How can I improve that and prohibit the bounce effect in a more "elegant" way?