I have a 454 engine in my cabin cruiser boat. After operating at normal cruising speed for 20 minutes or so a spark plug was blown out of the engine, with the center electrode melted, and the rest of the plug left in place. I replaced that plug, checked the others and they were fine. The next time out, the same cylinder, brand new spark plug blew out again, same deal, melted electrode and the whole center of the plug blown out of the engine. It has a mercruiser electronic ignition. Any Ideas??