I had a similar problem with the Evolution 46 trainer engine that was in my Alpha 40. I bought it used and the engine had not been run in a few years. When I talked to the techs at Horizon Hobby they said it was bad berings. I sent them the engine and they replaced the berings and sent it back in a week. I don't remember the cost, but it was cheaper than a new engine and I haven't had any problems since. They sent the old berings when they sent back the engine, and you could cleary see I needed the new berings.