I'm late to this, but the key to good shifting are correct trans pressures, and that's controlled by the modulator plus the 3-position solenoid. You can hook up a pressure gauge and check them, and will likely find they are off.
The BBB lists the pressures, but I don't recall them off the top of my head. There are three pressure setting, controlled by the 3-position solenoid. The higher the pressure, the harder and more abrupt the shift will be. Also the higher the pressure, the higher the engine will rev before shifting.
Low pressure is for down shifting so you don't get whiplash. Shifts are slow and soft.
Mid pressure is for normal driving. Shift are distinct, but not harsh.
High pressure is for hard driving and will typically cause a downshift and hard upshifts.
The three pressures are controlled directly by the 3-position solenoid, and this is often when problems occur. When depowered, like with the ignition off, the solenoid is in it's mid position. You should be able to grab the linkage rod and move it forward (high pressure position) and aft (low pressure position).
Now turn on the ignition and check for proper movement of the solenoid.
With your foot off the gas peddle, the solenoid should be shifted aft. Cracking open the the gas should cause it to shift back to the center position. Control is via a switch on the throttle body that closes when the throttle flap is closed. That contact energizes the solenoid to command low pressure.
Next check the high pressure position (forward). This is controlled by the kickdown switch under the gas peddle. When you depress the switch by flooring the peddle, the solenoid should shift forward.
With all that working, you can now do the same checks, but this time with the engine running and checking the actual pressures. In the three positions, you should see pressures consistent with what the BBB calls for (sorry, I can't remember the valves). If they are off, then adjust the solenoid linkage as needed. You may also find that the solenoid and linkages need to be cleaned and freed up to get full travel.
Making these adjustments on my car transformed a seemingly junk transmission (slipped and wouldn't shift) into a like new transmission. No parts, just time.
