Fluctuating neg. to ground on ignition coil

Please don't get carried away on ballast resistance. Much better to measure coil + and it varies all over the place. But at a steady RPM the voltage should be steady. It varies from somewhere around 8 with engine stopped and key in run, to as much as 12 or a bit more when the charging system is at 14V

Ballasts are cheap. Frankly if you really are convinced you have an itermittent somewhere, and YOU ARE CERTAIN it is not the wiring path---which is far more likely, then the only real solution is to pick the cheapest parts and replace them

And this comes from a guy who does not like throwing parts at a problem

What to do in the meantime?

With engine stopped, key in run, monitor the voltage to the "key" side of the ballast. It should be "same as battery" but often is not, due to wiring problems/ voltage drop in the harness

Wiggle test the harness, especially the ballast and bulkhead connector

Remove the ECU scrape around the bolt holes and remount, using star lock washers. Same for voltage regulator. Both MUST be grounded to same potential (voltage) as the battery. MUST

Check all connectors in the ignition system. Work the ECU connector in'/ out several times to scrub the terminals and feel for "tightness and inspect with a light for corrosion, same for ballast terminals and same for dist. pickup connector

If you can beg borrow or otherwise get an ignition or any other kind of oscilloscope, taking a look at coil neg may give some clues.