Disconnect regular power and jump direct from battery. See if that corrects.

Do not leave jumper hooked up for too long. I would not leave on for more than minute or two.

6 or 12 volt? Do you have a ballast resistor?

If corrects then coil distributor and all that should be OK and you have a problem in wiring between coil and all. First place I would suspect is worn ignition switch. Check all connections for tight and clean and good crimps.

If you have VOM do a wire by wire resistance check. All readings should be 1 ohm or less except coil.

Dan Bentler