Classic/Antique Car Repair: 1967 289 spark problem, pertronix ignition, exhaust system

Question
We have had the Pertronix ignition for a year. Seems to be missing on a cylinder (or more) once warmed up. We replaced the dist cap, plugs, and thought it was fixed. Ran great for a day, then mystery missfire begins. One old plug looked like it had not been firing for a while when we replaced them. Think Pertronix is failing?, or a bad plug wire (those are lees than year old).

Answer
I have no idea what you are talking about.  You put this system on a Model A????  
Sound like it's a heat related problem.  Check to see what is near the exhaust system that could get hot and warp or stop working.  The electronic box that controls the system could go bad when it gets hot.
Sorry, but I am a Model
a guy and don't know your system.
jack