Just a couple of thoughts. First, a vacuum advance will not cause a misfire unless the advance is stuck in the advanced position. You say the timing is correct. That is done at idle usually with no vacuum applied, so I don't think that is the problem. For example if the base timing is 0 deg and it is set at 0 deg then the car will run fine without the vacuum advance but use a lot more fuel at highway speeds.
Second, I believe that engine has electronic fuel injection. If it does, there are i believe 4 sets of points below the advance plate. These points are used for fuel injection timing. If they have a problem you will have poor performance.
Does this engine have ignition points or electronic ignition?
I may be all wrong about that car. It has been a long time since I have worked on one that old but the part about advance timing still stands and applies to all cars.

Paul

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84 500 SEL (307,xxx miles)