Just about every CM-6 plug I've ever seen comes out of the box gapped at .018". Unless it was dropped on the end at sometime. That's what NGK tests them at. If you started out with a closed gap CM-6, you might think about using another plug. Good idea to check them when you purchase them if you're at some place where an exchange could be made. If a plug was dropped the ceramic could be fractured. Sometimes you can see it, sometimes you can't.

As things stand you maight want to go through all the connections on your ignition system. An NGK CM-6 runs almost forever when gapped at .018", so you have other issues in play.