I had one of these and it was stable in harrier. Do you have your C.G. near the rear of the recommended range? I used an o.s 1.60, and the plane was very light with this engine. If you are getting a lot of wing rock one of 3 things could be the problem.

1. The plane is too nose heavy.
or
2. The plane is too heavy for the amount of wing area (wing loading too high). This shouldn't be a problem unless you are adding weight or your engine is very heavy.
or
3. You don't have enough throw on high rate elevator.

There is a range of elevator throw (generally between 20 and 30 degrees) where the plane gets squirrly, but after you pass this range and get into the 35 to 45 degree range, the plane will want to lock into a harrier. A small amount of wing rock is normal but
this can be countered by aileron.

Greg