[Don]

What inside the distributor would cause only the same cyl. To misfire???
The rotor not getting spark to the cap prong or cap contact,,,if rotor to contact is to great it would probally fire intermintally,,,what would cause gap to change??
Possibility the bushings,shaft could be a little off!maybe one lobe on shaft cam could be worn down just enough to cause misfire,,,,,any of these things would cause only one cyl to misfire,that is assuming he has checked the wiring and caps ,plugs etc. which he say he has,,,,I say rebuild the distributor COMPLETELY,,,!!