Intermittent Misfire

Double check the dist. carefully around the trigger device, the special device that is on the shaft for Pertronix as well as cleanliness, rust, debri around the module/ pickup and for distributor shaft wear or wobble

See if you can round up an oscilloscope, either automotive or even electronics bench type.
You can monitor the coil NEG and look for dropped triggers. It's possible it's a bad coil, something in the trigger device, or the module, or even a power issue. That last is easy, jumper battery power direct to coil+

Check plug wires for continuity and carefully look for any grounding points along wires or boots. I would try a different plug in the offending cylinder(s) even if you have to move/ exchange plugs to see if the problem goes to a different cylinder

An "open" or a "short" (such as fouled) condition can cause similar problems at times