Sounds to me like it is pre- igniting. With a glow engine the compression ratio, fuel nitro content and heat range of the glow plug are what dictates the "Timing" By using too high a nitro content and/or too hot a plug in a euro built engine can cause pre-ignition. Running fuel with high moisture content will cause the engine to run hotter and can be a cause of pre-ignition.



If I were in your shoes I would put in a medium heat plug ( OS #8 ), fresh 10% fuel and a 10X7 prop. If it still vibrates alot, you can try a second plug washer to reduce the compression some. One other trick that sometimes helps is to put the engine at top dead center and put an un balanced prop on it with the heavy blade exactly 180 degrees opposite of the head. This can fix an engine that has too little crankshaft counterbalance.