Auto gear box

Hi iback againwiththe faultyautogearbox on 2011 scout octavia .when igot the car the 4 wheel drive didnt work so the garage put a new pump on that seem to cure that but scince then the auto  gear box has developed a faulr it drives ok then suddenly the display goes from d 3 or d4 to d the car loose drve and free wheels the garage tookit in and said it was the pump on the rear axle  that hadnt been change right theyput another one on then said it was the computor? That controlled the 4 wheel drive they replced that said everything was ok . I had the car for 2 days only drove about 20  miles and it did it again it went from d4 to d i managed to get the car home but now it wont crank over it shows p-n or n-pin the display and i cant get it out of park any ideas