See if AutoZone or someplace like that can test your ignitor.

Also check for wiring problems at the distributor. The wiring harness plug can have dirty/loose pins or maybe wires that are rubbed thru & occasionally touching ground. Stuff like that...

Since it only happens occasionally it's gonna be tough to really catch it screwing up during a test.

What year/version? Not sure whether that'll make any difference, but who knows?

Is the coil internal (inside the distributor)?