Normally I'd say that you are using cheap plugs but the OS's, especially the #8's are great and normally last a long time. You mentioned 15% fuel, what kind? How much and what kind of oil is in it? Also, what are you using for a glow plug battery? It is possible you have an engine problem. I've seen a lot of Evolutions at the field that were a handful. Make sure all of the screws are tight on the head, back plate and carb. If the experienced guys at your field can't figure it out, I would make a call to Horizon and ask about having the engine checked out and repaired or replaced. Their customer service is always great.