Missing bad under load. Where to start???

ANYTHING in the entire ignition could cause this, from low primary supply voltage, to any deterioration in the hi V. end.

I'd be tempted to swap in as much different ignition (the entire system) as you can, and if it cleans up, THEN you know that it's "in that system" somewhere and can start looking. Be absolutely sure to verify good primary voltage.

Could be as simple as a bad coil wire, plugs, dirty/ carbon tracked cap/ rotor. Rotors can be tricky. I've see (black) rotors "punch through" and you could not SEE it without a strong light and magnifier