Sounds like you've eleminated the driveline as being the problem. I would highly recommend pulling the backplate off the engine (make sure piston is up) ck. for play in the rod bushing, also ck for loose particles that look like red rtv silicone. Some engines have had problems w/ the filler on the crank flaking off & getting into the engine it actually get's stuck in the glow plug electrode & fouls the plug. If you find nothing you may have a tuning or air leak issue. It only takes about 10 mins. to pull the backplate. If the engine is fairly new i would talk to Chad at team epic before i took engine completely apart. Keep us posted & good luck.