I did a couple drain and fills and at around 300 miles I get a little bit of bucking intermittently, very similar to a clunk, when cruising at 60 to 65 mph while in cruise control. After fluid change I did all city driving till today.
I read somewhere that changing fluid can sometimes cause the transmission to buck a little, and that there is a relearn procedure for the transmission.
I only have 84k miles. Car has no codes, fluid was a little dirty but not burnt etc.