Try moving the springs on the horns (on the rudder) closer to the rudder. In other words, lessen the amount of movement of the tailwheel from side to side. Tail draggers need very little in the way of movement. Using too much travel allows the airplane to get away from you during ground steering. In fact, you can disconnect the tailwheel from the rudder, and lock it straight ahead to check this. The Cub rudder is very effective, and normally has a lot of travel. From the photo, it looks as if you have a 1:1 ratio of the rudder and the tailwheel. Trust me on this, you want very little tailwheel movement. I only fly taildraggers.

Bill, AMA 4720
WACO Brotherhood #1