Dany's Magnum swap is back!! with Ignition woes now..

These can be tough to find, simple in the end. Simple as a coil going bad

Go through the ignition "one end to the other"

You said plug wires are old. Start there, you need them anyhow. Check resistance of them with an ohmeter "just to see". ESPECIALLY check the coil wire

Inspect rotor cap condition. Check for moisture, debri, carbon tracking, and check the rotor CAREFULLY and UNDERneath for spark "punch through" to the shaft

Change the coil......This is a shot in the dark, but coils do this, and there is NO WAY to test them other than try another

Check and improve the grounding on your ignition box. Clean/ scrape around the holes front and back, and the holes on the firewall, remount with star lock washers. Pull all connectors, inspect with lamp for corrosion, etc, and push/ pull them all in/ out several times to scrub the terminals.

Then check voltage. with engine off, and key in "run" check ballast resistor voltage each side. "Key" side should be "same as battery." Coil side will be much lower perhaps 6-10V Again, there is no way to check the coil

Suspect your tach if you have one. Disconnect everything from coil NEG except the wire from the box, and see how that runs.

And last, suspect the plugs

NEXT
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A flat cam can cause this along with certain carb / fuel problems.................................