What could it be?

I drive a 2001 Pont Grand Prix. I occasionally experience a hard shift, when the car changes gears. There is not a delay in the shift, however, it shifts hard.

Did you check the transmission fluid level and condition? Has the transmission been maintained properly all along?

Is this when the car is cold, warm, or both?

check transmission fluid level and condition seems good

happens cold and warm, but only at times never know what brings it on