pops out of gear while driving

While driving it will pop out of gear. Then have to pull over to side to put it into park and then into first and then into drive then take off again down the road. This lasts for a while and then it will repeat itself. The neutral safety thing that mounts near the bottom of the engine was replaced and the problem did not reoccur for about a year. It is now popping out of gear again. The most recent time it happened was while driving on a cobblestone street. This type of street caused a lot of vibration. That is about all I can think of.
Thanks for your help for my sisters vehicle problem.
Jim Felland (Hughlene Dunn is my sister)