I had the same experience with mine in the beginning. It purred like a kitten after a few tanks, and seemed to run really well. I used the suggested 18-8 prop, which I think is way too big and heavy for this engine. My problems didn't show up until the flight testing. I'm curious about something. Do you have the engine mounted to a table? Or is it mounted to a portable stand that you can pick up and change the attitude with? I'd really like to see what your engine does when you raise the nose at 1/3 to 1/2 throttle after both needles are tuned to your liking. Mine would sag right down and die when doing this, but I found out first in flight. My engine was mounted to a Funtana 90, so there wasn't much full throttle flying unless I was going vertical. Even then, it was not necessary.

My favorite props are the 16-10 for speed and penetration, and the 17-8 for harriers. I tend to like to fly my Sukoi more like a full scale than a toy, so I keep the 16-10 on it.