2002 Volkswagen Passat hard shifting from 3rd gear to Drive

Last week the transmission shifted really hard from 3rd into drive. It was a hard bang. It did it a total of 3 times and only when shifting into drive from third. I thought that something big time was wrong with the tranny. We were on our way out of town for a vacation. I went to pick my wife up at her work and then leave town. When I picked her up I shut off the engine and went into her work to get her. I informed her we were not taking this car because I was worried it would not make it. We went over to our family mechanic who is not a VW guy and he put a scanner on it and it showed some misifires only and he cleared the computer. We then went to a tranny shop and he would not work on a VW because he was not trained. Well after driving around town for over an hour I realized that the transmission had not banged at all after I had picked up my wife. IN fact she thought I was making the whole thing up. We took a chance and drove the car from Phoenix to Salt Lake city and back. Over 1500 miles and the car ran perfect. My only thought is that since the shifting is electronic that when I turned off the car when I picked my wife up that it reset ( just like a computer does when you turn it off and then back on) Do you have any idea as to why the tranny shifted hard and then stopped and ran fine. I don't want to be stuck someplace in the future because I failed to follow thru on what caused this in the first place.