2001 BMW 325 intermittent hard shifting

Sometimes and this can happen only once a week, month or longer, when transmission is changed from Park to Drive, a hard jolt is felt and as the car starts to move and shift into higher gears, the gear change is hard accompanied with a jolt as it moves thru higher gears until vehicle reaches full speed and the same heavy jolting and jerking happens when the automatic transmission is downshifting when the car is slowing down. If the vehicle is parked for sometime and then driven, it drives fine as if nothing had happened. Auto transmission fluid-engine oil are all OK. Thanks for your help !