iPhone Roll A Ball Variation question

Unity Engine
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1

I am playing with the Roll A Ball project and trying to replace the ball with a sliding cube.
I want the cube to turn and point in the direction it is heading.
I have been playing with this code:

var targetRotation = Quaternion.LookRotation (rigidbody.velocity);
 	transform.rotation = Quaternion.Slerp(transform.rotation, targetRotation, Time.deltaTime * rotationAdjust);

But I am getting erratic behavior with the cube rotating all over the place.
I am only concerned with altering the left/right orientation of the cube through the script. What would be the best approach for fixing this?

2

Do you know that your velocity vector is only in that flat plane and never includes any up/down component? What exactly is happening that you deem “erratic”? (is the erratic behavior in the left/right plane, some up/down parts, other, etc.).