The solution may be related to a bad engine ground. Your high voltage ignition coils jump across the spark plug looking for ground. IF the engine ground cable or bolts can't give a good contact to the battery ground the PTO clutch could. 

While not easy, the high voltage (12,000 Volts) is enough to arc through the crankshaft into the windings of the Clutch and back to ground. 

This is not a 100% answer but a direction we are looking to test. IF you test and find it true please reply.