ignition circuit problems

[Justin 66GT]

Hello all, newbie here. I figured I might as well make my first post a doosie...

The previous engine (original 289) had an aftermarket ignition consisting of a Mallory Unilite distributor and Promaster coil. It ran like this for over five years but did burn out an optic module early on. When it was replaced I was told to add a ballast resistor on the ignition wire going to the + post of the coil (even though the stock wire is supposed to be 1.3-1.4 ohm resistor), and it ran fine this way until I pulled the tired 289. The new engine has a new 351W Unilite distributor, using the same coil. Spark is not constant; if I put my timing light on the coil wire and crank the engine I'll get no more than 3 consecutive sparks, with many gaps inbetween.

The Promaster 29440 coil tests out to .6 ohm primary resistance and 12.41k ohm secondary resistance, supposedly to spec. Battery voltage measured 11.85 volts. Ballast resistor on the ignition wire measured 1.1 ohm.

With everything connected up (resisted ignition wire and red distributor wire on the + post of the coil, green distributor wire on the - post, and brown to ground) and the ignition turned to ON, I measured 7.43v on the ignition post of the starter solenoid (marked I), 7.22v on the input side of the ballast and 3.5v on the output (+ post) side. With the ballast resistor removed and everything connected, I measured 4.6v on the + post of the coil and 4.8v on the I post of the solenoid.

If I remove the wires from the + post of the coil I get full battery voltage on both sides of the ballast and the I post of the solenoid. Same thing with the resisted ignition wire directly on the + post and with the red distributor wire loose, and vice versa (ignition wire loose and red distributor wire on + post).

If both the ignition wire and the red distributor wire are loose from the + post of the coil but held together, I get 11.75v on the output side of the ballast resistor and full voltage everywhere else.

I am unsure what would cause the voltage drop in the circuit with the wires connected up to the + post of the coil, and why the voltage is lower all around with the ballast resistor removed. Would a faulty ignition switch be a culprit, or should I suspect the old coil even though it tests ok? TIA for any thoughts or suggestions.