Transmission jerking / stalling
 

I have a 2008 Cayenne with 100.000 km on it. Every so often, the transmission behaves very strangely. While driving at around 90Kmh I will accelerate f. ex. to overtake traffic but instead of going faster, my car will jerk very violently. I release the throttle immediately because it really feels like something will break. I can't figure out under what conditions this happens but it is not regular and does not have to do with temperature (ambient or transmission). It really has little to do with speed either as this may happen driving in the city. Any idea what might be happening?
Something else that might be related. I was recently driving along the highway when suddenly my car acted like it was in neutral. I reved up and nothing happened. I stopped the car. Put it in Park and then back into Drive and every thing was fine. Are these incidents related?.
Thanks,
Ólafur