Transmission Not Go Into Gear problems of the 2005 Mercedes Benz ML350

Occassionally while driving the vehicle, it stalls in gear,that is the gear shift doesn't work and the vehiclestays in one gear even while moving. Usually the driver has to stop briefly shifts the gear to another gear and then starts again. In a few seconds wwe can drive again. Until the same thing happens again perhaps in a few months or so. This always happen when the vehicle is in motion more generally on city street. This began to happen from the very moment I purchased the car and within two hours of driving and has continued to happen since.