Hello, welcome to JustAnswer.
One to one and a half inches of play really isn't very much by boat standards. A brand new boat can have nearly that much.
There is not adjustment, but you can make sure the big nut on the transom end of the cable is tight. Then go to the drive and move it back and fourth by hand while watching the steering wheel. If there more than 3 inches "wag" without the steering wheel moving, your gimble ring may be wearing. With only 100 hours it shouldn't be unless the boat is driven with the drive up beyond the trim limiter. In which case the repair is extremely expensive. ($3000 or more)
What you need to do is make sure your boat is loaded evenly. If your boat leans to one side when you are sitting at the helm, that will cause wander at Idle speeds. Other things that will cause this are a drive that is mounted crooked in the transom, or an uneven or un-true keel.