If you are getting more than 2 flights out of your glow plug on 20% nitro, you are not pushing hard enough. I presume that being you are running a prop of less than 7 inches, you are going for maximum speed from your plane like in racing. It is not uncommon for engines being run hard to also be hard on glow plugs. I used to only get 2 to 3 flights out of a plug in my K&B Cox Conquest on 10% nitro turning in excess of 26,000 RPM. The prop was a 6 5/8 by 4 1/4 hand laid glass prop.