I would like to hear where you ended up with this autopilot problem.
I also suspect that water speed over the rudder is the problem.
I also have an EVO100 linked to a hydraulic ram and a rudder reference unit and have never, ever had such a good and easy to calibrate auto pilot. From almost straight out the box there was almost nothing to do other than tell it i am a sailing boat and want it to react in cruising mode rather than race mode. It can also hold to a point of sail if you have a modern wind anemometer and will auto tac but it does tend to go too far and then come back. If linked to a MFD, it gets automatic deviation feed, and according to the literature , it learns your boats handling characteristics.