Its a YAK. Yaks typically do not have significant issues with wingrock, as a matter of fact they are probably the most stable 3D platform there is, IMO. I have had Edges, Extra and Ultimates but have not owned a Yak, however I have flown my friends misc variations. The Yaks to me are as stable as you can get.

I would check and make sure that your lateral balance is good and form there its just a matter of sticktime. The plane should basically be trimmed out for normal flight and coupling issues that could not be corrected by setup should probably be mixed out. Yes, coupling effects the plane in every plane, not just on knife edge. SO the mixes will subtly correct some problems you are having transitioning from harrier to hover.

My bet is that you are behind the airplane a little. All planes wingrock some, but you cannot tell when a pilot that has mastered the harrier is flying because he/she is subconsciously correcting before the problem ever starts. If you are looking at his sticks, the corrections are so small that you cannot see it usually. If you ever get up really close to a "pro" 3D pilot, you will be amazed at how little the surfaces move when they settle into a harrier or hover. It looks like the plane is just hanging there motionless, and it is, but that is because the corrections are way ahead and therefore they are VERY subtle. I have actually had a pro fly my Extra and although I wrestle with it some, in his hands it looked to be an absolutely perfect and stable plane. It would hover motionlessly inches above the grass like it was hung from a string. Practice makes perfect guys.

Think of it like riding a bicycle. You are CONTSTANTLY making tiny corrections to keep the bicycle upright without even thinking about it. When you 1st start you either over correct, or do not correct soon enough and the bicycle is all over the place. Eventually your brain figures it out and you ride the bike without ever realizing you are correcting, much less seeing visible corrections. In time you will just do it. You will harrier and hover the same way.. Just keep doing it and eventually it will be second nature.