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Not that I am an expert, but just after gathering info from the internet..
How was that 20/30kV measured? With the engine running that sounds way too high, the spark should occur below 20kV.
The coil only has one 'floating' secondary winding. The high voltage to the other plug is positive and to the other negative.
Under heavy load the required voltage is higher than under normal load. If you have bad plug wires or boots, you may have arcing to ground and no spark under heavy load. So a second vote from me to the diagnosis of OlHoss.