Automatic Transmission problems

I have a 2000 Buic Century, after driving a while in the city the transmission starts to shift very hard. As it changes gears it slamms into each gear. It does not do that when down shifting though. Doesn’t always do it, mostly when I have been city driving for a while.

Is the transmission fluid level correct? When was the last time the fluid was replaced? How many miles on the car?