Stan, While all this seems neat, where does it lead? There are no schematics to isolate a component on the board as an issue. Wouldn't it be easier to pull all the harnesses except the 7 pin (power and tach) and the 5 pin (motor) and see how it runs. This eliminates those possible grounds in lighting, smoke and sound. If the engine runs fine, then install harnesses one at a time until problem comes back.
If still bad, replace the tach reader, though this should also cause sound issues. If still bad, and you are sure nothing mechanical, then replace the processor board. G
