transmission shifting/banging hard only after 20 minutes of driving..why? on 2004 Chevrolet Monte Carlo

transmission shifts hard after 20 minutes of driving the car especially if im at a red light or stop sign then i hit the gas to go it bangs into first and second gear its awful

Asked by for the 2004 Chevrolet Monte Carlo
get to tranny shop for diagnostic. tranny tech. will advise.