Since the problem occurs with two completely different trans, and the transmission is electronic, I suspect input signals from various sensors/pcm that tell the transmission when to shift. Possibly faulty wiring, bad connection(s).
Throttle plate closed, a 32 mile per hour 3-2 downshift seems a bit high. Check the vehicle speed sensor, throttle position sensor, and mpls. Again, check for faulty wiring, and connections.
1995 Thunderbird LX 4.6l 97 Headlamps/Hood All Red Tails Muffler/Resonator Delete
2004 Tahoe 5.3 4x4
2008 Acadia Limited AWD
...still looking for a clean, low mileage, rust free 96/97 Thunderbird 4.6