I had exactly the same thing happen to one of my gassers. Turned out the jomar electronic switch for the ignition went bad. Apparently the e switch would only allow enough current through to run the ignition at idle speed. I proved it by taking the switch out of the circuit, then fixed it by replacing the e switch.

Yes, a bad mechanical switch or any other component with high resistance or intermittent contact can cause the same problem. If you have an intermittent connection, the ignition will fire every time the circuit is either broken or made (not sure which). Under that condition the plug will fire with no regard to the crankshaft position.

Jerry