I had a car that had a clunky tranny and I took it to the dealer and
they put some additive to de gum the transmission.  Took about a week
or two but things became smoother.  Don't know what the product was
they added.

You need to check the vacuum hose going to the tranny on the driver side.
Make sure the connection is good and tight.

If you are original owner and never had the tranny overhauled, then it has
to be vacuum problem.

If overhauled or not original owner, then check vacuum hose and you may have
to adjust the vacuum modulator on the side of tranny where that vacuum hose
goes to... by turning to the front of the car direction, you increase the
vacuum which soften the shift. Adjust a 1/8 of a turn at a time and remember
your original position.