I think I suggested the test idea but it is not a perfect test, because ignition is more difficult at full load. One reason is the dynamic compression is less unloaded because the air limitation by the throttle. The more compression, or more the fuel mixture is off the harder for ignition. The higher the RPM the less time is available to charge the ignition coil, and a weaker ignition when needed the most.
At 6,000 RPM the time between cylinder events is only 2.5mS. In that time the coil needs to be charged and there also needs to be time for the ignition event, it takes some time. The coil charge time or dwell is related to the voltage level of the supply, the inductance and resistance of the ignition circuit. To get the best performance requires a good supply voltage, low impedance coil. Ignition systems that do not have active dwell control (points or OEM mopar box) require a ballast resistor, this is a disadvantage. A mopar chome box helps but it requires the application to be racing, or there may be reliability issues of box and coil on the street. The GM HEI ignition actively controls the dwell, it does not require the ballast, so a shorter dwell is possible for more high RPM spark energy.
To get serious about diagnosing ignitions requires a scope and special voltage and current probes.
With the number of variables at play it is sometimes necessary to be a scientist and detective to sort things out.
