Cranks but won't start. Unplug ground at field spade on Alternator and it starts?

I have ohm'ed the Ignition 1 and Ignition 2 circuits from ignition key plug to coil. They were zero ohms. Positive battery cable to ignition key bat terminal zero ohms. Alternator to starter relay post zero ohms. New ignition switch tested before install: zero ohms when key in run and start on applicable terminals and open circuits on applicable terminals. Zero ohms from voltage regulator and alternator to neg battery. Two week old ballast resistor .8 ohms.

Once car is running amp meter shows strong charge, lights are bright, car runs perfect. After driving a while amp meter stays on slight charge lights off. So charging systems seems to be working perfectly when car is running. Only symptom is no spark when cranking due to low voltage at coil.

Car started perfectly for two weeks after new alternator and voltage regulator were installed. Even drove around with AC on for two hours in 100 degree heat charging systems was good and strong. Stopped for gas last friday, no spark when cranking engine over issue ever since. Starter seems to turn the motor over fast and no signs of starter weakness or dying off while cranking engine. Mini starter been on there for 20 years or so.