Headlight switch doesn't care if it is 12 volt or 6 volt.

Could be the condenser in the distributor is heating up and breaking down.

So I understand you have put it back to 6 volt? Did it have a ballast resistor in the line to the coil when they tried to convert it to 12 volt? If so is it still in the line? If it is then it needs to come out. 6 volt coils don't require the resistor in the line.

Could you have a fuel issue instead of ignition? Sometimes if the vented gas cap is stopped up it will run a few miles and then either die or run bad because it is pulling a vacuum on the fuel tank and can't pull enough fuel up to the engine.