A pix of the steering cable pulleys and how the cable is run would certainly be helpful. Having just been on someone else's boat yesterday that had horrible geometry for his cable setup, it could be how/where the cable's fixed end is attached or the spring tensioner for the cables.
My friend added a jackplate and then added a modified steering arm extension that was way too long. Centered, the cables were adjusted correctly, but he'd eliminated the spring. When turning either way, the big ole arm swung closer to the plane of the last hull mounted pulleys, and introduced quite a bit of slack into the cable on 1 side or the other depending on which way the motor was turned. Cable kept jumping off the steering drum behind the dash. Not good at 30mph+/- running down river in a fairly strong breeze in a lightweight tinny.......