An Evolution engine that is running too cold will have all of the symptoms described. A good way to test these engines is to watch how they run on the first crank of the day. See if you can get it to accept the throttle by advancing it gently. I've had to back off, advance, back off some, etc until the crankcase cleared. I got pretty good at it half of the time before W8YE gave me a hotter plug to try. If it is a plug problem, you may be surprised at how much more power you will get out of the engine, and the smooth throttle response.

I'm not sure now if the ones I've been around shipped with the H9 plug. I always pull them after some amount of break in time.