Mopar Orange Box with Tach

The first thing to test is the the voltage at the + terminal of the coil to ground. With the engine running at idle and just above 2500 RPM. The ballast drops the voltage at that point, it will be typically about 7V. The charge time of the coil is directly related to the supply voltage. At low engine speeds, the available dwell time is long, as RPM increases, the available dwell time decreases, and the coil is not getting charged enough for proper ignition.

If my hunch is correct, there will only be 3 or 4V. If the voltage is correct then measure voltage from engine ground to chassis, it should be near zero. The root cause may be a poor connection at bulkhead connector, or engine to firewall ground.