rudder rub
The first thing to do is when the boat is at rest, swing the tiller back and forth and see if the rudder movement directly matches what you are doing with no slop or play (there may be a little play at the tillerhead, though). You are looking to see if the rudder movement in the water matches what the rudder post is doing from in the cockpit. Do it quite vigorously to match high loading when sailing. You should be able to tell if the rudder blade is moving independently of the tiller.
Also check to see if there is any significant friction when you do this (at the dock)-to suggest the rudder is rubbing against the hull.
Let me know what you find (see my profile) and we can go from there.
Happy T-day!
Seth