Gremlins in my Demon...........??

You may have to take a look at the electrical part of the ignition switch at the bottom of the steering shaft as they can get out of adjustment as the rod that actuates it comes from the key part of the switch so a simple adjustment of the bolts and sliding the switch and your done. Just another area to look at. As for the plug it generally melts it and shorts the wires together and fry's the grounds. Or it could be a grounding problem at the engine, or from the engine to the chassis.