Can you say "Induction Heater"? Another quick question just to throw out a possibility......are you using different terminals on the amplifier when wiring one way versus another? If so, you could very well have a dead terminal usually due to a shatty/cold solder joint on the board. Very rare, but possible. I'm just asking since they've covered the opposing coil polarity possibility and since you said it read 4ohms on your testing equipment, so your terminals or connectors between coils are not the problem.