Well I have only worked on 1 of these cars about 10 years ago. I am just trying to remember what I can and passing that on to you.
The vacuum advance should hold vacuum but still I don't think that will cause the lack of power. When you push of the go peddle vacuum drops so you will have no vacuum advance or retard. In other words timing will be at base or advanced some by the mechanical advance.
Another thing I remember. Under the dash on the right side there is a fuel computer that has a round black knob on it. The knob will adjust the fuel mixture. I don't know how much but I remember adjusting it for a smooth idle after I got all the other problems taken care of. It could be running a little lean or rich and causing the problem.

Paul

__________________
84 500 SEL (307,xxx miles)