On ANY model engine, blowing a plug per flight usually indicates a too-lean condition, too much heat, or an over-rev. A good-quality plug should easily take the RPM, so I doubt it's that.

Most likely, the engine's running a bit lean, and you're getting some detonation. Examine the glow plug element. If it's being beat-up and deformed, detonation is the most likely culprit. If the element's gone, it could also be too much heat.

An engine will detonate if it's too lean, if the compression's too high for the nitro, or the tuned pipe is too short for the RPM. To much detonation, and you can burn a hole in the top of the piston.

Pull the head and look at the piston. If the center of the piston is starting to look like it's sand-blasted, and you can see a ring on the piston top that corresponds to the squish band, then you've confirmed that detonation is happening.

The cure is to run richer, add head gaskets, lower the nitro content of the fuel, or lengthen the tuned pipe assembly...or any combination of these.

The geared engine combo turning near 14K is really working hard. The engine would tend to run warm because the heat of the gear train would be transmitted back to the main case of the engine, which would be hard-pressed to keep cool. You need to make sure you get very good aiflow through the fins of the cylinder and head. A lot of airflow over the gearbox helps a lot. Using fuel with 20% oil or more will also help. The extra oil gets thrown overboard, taking a lot of heat with it.

Hope these ideas help out.