If you have a metal boot on the spark plug try a hose clamp around the base of the metal cap. You can get them at an auto parts store. They are about 1/4" wide and used for fuel line's.

I had a horrible glitching problem on a 60cc engine. It would only glitch at mid throttle. WOT and idle were fine. The metal cap on the plug was just loose enough to vibrate against the spark plug and cause metal-metal noise that drove the receiver nuts. Only way I found it was by getting the engine at the right rpm to cause glitching, then twisted the metal boot back and forth. A little hose clamp fixed my problem for good.

It's very easy to try and cheap. Hose clamp is about 50 cents.