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
1 answer
get to tranny shop for diagnostic. tranny tech. will advise.