Many years ago, I developed a technique to cure beginners from erratic flying. What I have them do is look down at their fingers on the control lever while they're flying. It seems that when they are looking at the plane, many aren't aware of what exactly the attitude the plane is in. Their brain is conflicting on what they think their eyes are seeing and what I am telling them to do. For example, their eyes might be telling them that the wings are banked in a left turn, or not even have any idea what is going one, and therefore, in their minds, I am giving them the wrong command to make the appropriate control movement.
So, by having them not look at the plane at all, they will have no choice but to follow my commands to move the stick exactly when, where, duration and speed of deflection. This have proven many times to be a literal instant cure, although sometimes just a temporary solution. I have them do this for a couple of minutes (more or less), and once I have them look back up, they continue to fly very smoothly. Also, they get to see exactly how little stick movement it takes to fly very smoothly.

I am very much aware that this is something that may not feasible for those who use a buddy-cord or pass the box method, but those who use a similar teaching method that I use might be able to eventually work it into their teaching technique. Then again, if the buddy-box instructor has a helper to work with the student and instructor, they might be able to try this "under the hood" teaching method.

CCR