2000 Chevy Impala chugging as it changes gears

We just bought a 2000 Chevy Impala with 65000 miles on it. Supposedly, we it had all of its fluids checked. Taking it into the high mountains it begain to chug every time it shifted gears. It now has 67000 miles on it and it is down on the plains. On long drives, 45 minutes or more, it still chugs as it changes gears. I will take it to a mechanic but I would like to know the probable cause so that I don’t end up paying for what I don’t need. According to the gages it isn’t over-heating? Why is it chugging?

Explain ‘chugging’ please.

No real noise, just a lurching sensation as it shifts.

Is check the engine light on?

No, engine lights or warning lights or messages at all.

Do you get a ‘clunk’ as the tranny shifts to a higher gear?

I don’t hear much of a clunk. The car is so nice and quite that it is hard to tell if the engine has started when I start it. The starting messages and warning lights come on and that is how I know that the engine is on. It is more of a hesitation before it shifts to a higher gear. Any ideas?

At the beginning of your post you mentioned the fluids had ‘supposedly’ been checked.

Follow the instructions stamped on the side of the trans dipstick and see for yourself.

If the fluid IS low, the trans will act up and a hesitation is possible.

Make sure the vehicle is on level ground (both side to side and front to rear).