You can completely eliminate the electrical by attaching a wire to the bullet connector at the solenoid and, either monitor the circuit for 12 volts with a test light, or connect it to an accessory wire to provide 12 volts. The circuit should be powered any time the ignition switch is in #2 (run) position and the upshift light is not on.

If it doesn't cure the problem; leave the test light or jumper in place and loosen the downshift cable a few turns and retry. Sometimes, the outer housing of the cable gets pulled out of the crimp and makes the cable way too tight. The throttle bellcrank should hit the limit of travel BEFORE the downshift cable is pulled to its max.

If none of this fixes it, you may have worn slip rings for the OD brake "B0".