My ideas all involve diagnostic testing. The possible problems are too numerous to expect "answers".
One simple test that will give direction is to remove the aircleaner and lightly depress the airflow plate while idling. If this makes the engine briefly smooth out you have a lean misfire of some nature. A pair of needle nose pliers can lift the plate (by the ten mm screw in the center) to lean out the mixture.
A car that has a working lambda system isn't likely to be running too rich. The usual problem is that a restricted injector (or fuel dist circuit) or vacuum leak causes one or more cylinder to go over the line lean. Remember that the lambda control system takes the average mixture to a very lean condition. Deviations rich are counteracted causing some cylinders to go too lean.
The advantage to this simple test is to see if your problem is mixture based. There are still a bushel of possibilities. They all are as exactly testable with proper equipment and technique.
One point of feel: a proper running feedback control system (lambda system) will correct for your actions so as long as the oxygen sensor is connected the results of depressing the plate (enrichening) are brief, although if in a few seconds the miss occurs a further depression should again smooth out the engine.
BTW one very common vacuum leak is the line going to the vacuum modulator. It can be seen looking straight down at the back of the motor. It is a white plastic line that goes under the exhaust cross-over pipe.
Bosch Master, ASE Master, L1
33 years MB technician