Look for any loose wiring connections, possibly a bad ignition switch? They get wonky when they get old.

Pertonix Ignitor likes a clean low resistance connection between distributor and ground. This assumes the standard Ignitor 1 (#1281 module). Here's how to perform a ground test:

GROUND TESTS
It is imperative that the power and grounds be checked as part of the installation procedure. After installing the Ignitor module and the distributor and with the distributor in the engine, use a digital multi-meter to measure the resistance from the aluminum plate holding the module to battery (-), the net resistance must be less than 0.2 ohms. (Set meter to lowest ohms setting). The net resistance is the meter reading minus the resistance of the meter leads. If the net resistance is greater than 0.2 ohms, the source of the faulty ground must be found and fixed. Usually the source of the bad ground is easily found by holding one probe on an original location and moving the second probe toward the static probe. Where the resistance drops identifies the source."

Right, but you're looking for an intermittent, that's the problem. Or the other problem anyway. My old truck one time shut down, just like somebody flipped a switch, out in the middle of nowhere at the goosenecks of the San Juan in Utah. I looked for black helicopters, didn't see anything though LOL. I snapped a pic of my truck, got back in and it started right up, never happened again.

I think it could be the ignition switch in your case. Try jiggling it around maybe, bang on it, see if you can duplicate the problem. Worth ruling out anyway.

Maybe try finger-faddling the wire harness where it connects to the ignition switch itself, or jiggling the key at idle, carefully calibrated percussive maintenance with a hammer, etc.

What brand of ignition coil is it? How many ohms? When things get hot a defective coil can go intermittent too. Are you running a ballast resistor? This makes for a hotter spark but the coil can get pretty toasty this way, and ignition coils are made overseas cheaply now. Intermittent electrical can be tough to find.

The ballast resistor is "baked in the cake" on Fords, it's part of the wiring harness. It's likely still there. NAPA has good coils. Now they make different coils for different applications. Just to rule this out, I'd want to know the ohms resistance on the primary windings. What's the part number of the coil, or do you happen to know how many ohms?

It would be a good idea to make sure the ohms resistance in the primary ignition circuit is not too high, nor too low. Also sometimes folks want to "hang" other items electrically on the ignition circuit because it's a convenient hot. Don't want to do that either.

Yeah, I noticed that after posting. Sometimes the only practical way is to eliminate things, find out what it ain't, and eventually whatever is left, that's it. Be sure to check the ground resistance at the breaker plate as mentioned. Pertronix works great but transistors don't like poor grounding.