It is so hard to tell what your problem is since you already trouble shot the most probable problem cause, the plug. (Of course, I hate break-in as far as plugs go, pluges become disposable items as the rich mixture destroys them. Spending $6-12 USD on plugs during break-in is annoying for me.)

Can you post a vid with sound of your RC as it is now? Also, run it up and down, some at idle, some at half trigger, and a full trigger pass at your current tune. This will give us an idea of if you are too lean, rich, and help us guess if an air leak at the carb, backplate, and/or bearing seal might be the issue.