You failed to say what year so this suggestion is for an early '80s
model. At the top rear of the engine is the transmission's pressure rod
(or cable) connection to the throttle linkage. If its disconnected the
transmission will up shift very quickly and at low speeds. Your car
seems to be the opposite.
The adjustment of the pressure rod (or cable) is to pop its ball socket
off of its attachment pin on the throttle linkage at the top rear of the
engine. Gently pull the rod (or cable) to its maximum and gently release
it. The resulting position will be its "natural position".
The adjustment is correct if you can then reattach the pressure rod's
ball socket to its pin WITHOUT moving the rod (or cable). But if that's
not possible rod's length should be adjusted so it CAN be attached
without movement from its natural position. If a cable, the adjustment
is made on the nut that secures the cable - the outer case is moved and
the inner core follows.