If the three cylinders are identified, a number of comparitive tests can be run. Onr is to swap a poor cylinders injector with a good one and see if the problem moves. We keep extra lines around so we can also change the firing order (bg) so to speak. Move the poor cylinder feed to a good cylinder.
Swapping the two distributors for comparison would be a very efficient testing move. I would have no problem just switching them and leaving it that way if the problem didn't follow. Be sure you are capable of setting up a new distributor before you attempt this. Its not just R & R.
Bosch Master, ASE Master, L1
33 years MB technician