98 Automatic Trans Jumps between 1st and 2nd

98 Contour V6, When engine is cold there are no problems. After the engine is hot and coming off the highway 1st gear jumps into 2nd jerking the car after stopping. Check engine light has been on for a while also. Help!

what is the mileage? maybe it is time for a trans fliud change?

Thanks for the reply. 82k miles. Had the transmission fluid flushed 5k ago.