I did not read every post, but I scanned them real quick. I did not see that anybody had this suggestion. I had a .61 that gave me trouble for a year. It stayed nasty all the time I could clean it real good and fly one day and it looked like it had never been cleaned. It ran good for about a year. Then it got where it was untunable. It ran (to the best of my memory) as you are describing. I bought it straight from horizon hobby, but did not save my sales slip. When I moved back home I became friends with the LHS owner. He gave me a receipt for around the same time I bought the engine. I returned it to Horizon for repair. They sent me a new one and told me that there was a hole in the case somewhere. Even though I could not see it. They said it was very small.

If you could get a receipt (if you don't have one already) and return it to Horizon I bet you will find out yours has a small hole somewhere in it.

My new one ran fine by the way.

Good luck and let us know what you find out.
Alan