Fluctuating neg. to ground on ignition coil

There will still be the resistance in the coil primary to limit current. When cold, the total resistance is around 1.5+0.6=2.1 ohms, and the coil by itself with the ballast bypassed for a short time should be around 1.5 ohms.

This test may tell you if the problem is something limiting the coil primary current.

And, BTW, coils can be tested but that is not a guarantee. They can be erratically breaking down and partially shorting inside. The only real way to determine if they are 100% OK is by substitution. The same applies for the ignition module; none of us had a good test for those, so we have to try a substitute.

The reluctor coil resistance can be measured when disconnected. IIRC, it should be in the range of 300 to 500 ohms. But I am not 100% sure of that number; maybe someone else can recall for sure.

And you ought to put your voltmeter on the + feed to the ballast from the ignition switch when running cold and hot, and see what voltage you have. The ignition switches can get faulty as well as the connections.

Let us know what voltage is across the battery terminals when idling at 1500 RPM or more. Measure that both hot and cold.