The automatic transmission system was designed to run smoothly when sifted to drive and transmit smoothly into other speed gears, however if your car develops transmission issues like hesitation, sudden movement or gear changes(slips).these could be the possible causes.
LOW TRANSMISSION FLUID PRESSURE: When this happens, the transmission torque is unable to transmit engine torque to the transmission because of low fluid pressure. Gauge your car transmission fluid pressure by using the dipstick. To ensure your reading is correct, do this on a warm engine. Besides keep your engine on idle while doing this so that fluid would be circulating and not settled on the transmission pan. A transmission issue could equally develop when a fluid pump goes bad or ceases to work by not supplying the right internal fluid pressure to make transmission smooth. Check too that there are no blockades in the fluid hose.
BAD OR FAILED PRESSYTRE REGULATING SYSTEM: the transmission system has a pressure control component (solenoid or valve). Have your mechanic gauge to determine what component is faulty