Make sure the muffler is tight (both the muffler halves and the engine screwss)...I had a similar problem with mine. You may even want to swap out the gasket and re-tighten...voila...no more issues.

This is a rather important thing to fix...when you are landing (idling for 5+ seconds) and get a bit of wind or a big bounce ...you need the smooth throttle transition to avoid pancaking it. Nothing worse than having the engine quit after you got the nose up with too little airspeed.