Tested the vcv today, and found that contrary to what I had previously thought, it loses vacuum at what Diesel Dan considers a proper rate, that is about 1 inHg per second.

Test drove the car today.

First, with mityvac teed into vcv output:
10 inHg at idle
8 inHg driving at constant speed
5 inHg with light pedal
0 inHg with 1/3 pedal

Second, with mityvac teed into vac amplifier's output to modulator on tranny:
12.5 inHg at idle
10.5 inHg driving at constant speed
5 inHg with light pedal
0 inHg with 1/3 pedal
My clunky downshift took place with about 7 inHg going to the modulator

Third, with mityvac hooked only to vac line going to modulator and with mityvac pumped up to 15 inHg as you suggested, Brian:
Tranny flared when shifting, and flared immediately before producing the clunky downshift yet again. Vac stayed at a constant 15 inHg the whole time. Tried to produce the clunk again and got it on the same downshift, this time with no flare coming first.
So, the clunky downshift is not the result of a low vacuum condition. Brilliant idea to test it this way, Brian. What does it mean that I still get the clunk? I assume that the downshift that is taking place is a 3-2 downshift. As I go from high speed to braking to 20 mph, the tranny shifts from 4 down to 3 I would guess, although I can't feel it. When I try to accelerate, the tranny kicks down to 2 unless I feather the pedal, and the shift control allows me to stay in 3 since I am not demanding much acceleration.

__________________
87 300D 162,000 miles: It's not just a car, it's an adventure!