95% of the time, blown plugs are caused by a lean run.

Id put the engine on a test stand, and try to duplicated the problem.

Most likely, what you will find is that the aircraft is at fault... even with a pump engine, fuel foam can be a killer. Id start by looking at the fuel system. Look for leaks, kinks, and a tight fitting tank that might cause the fuel to foam while in flight.

What you described may be an intermittend problem. But it only takes a temperature spike or two to cause a plug to burn the element.

A pipe that is set too short for the prop being used will also tend to cause the engine to go lean in flight, or to find a temperature spike. And on occation, the issue of temperature or the lean condition comes from throttling down, and trying to get the engine off of the pipe. The engine is at partial throttle, reduced fuel flow, but the pipe trys to keep the engine running at the higher rpm... demanding fuel that is not being delivered... thus a momentary lean condition.

Bob