Box shivers up and down

I made a box that i can push but is shakes
Can somebody fix this?

if(hit.gameObject.tag == "box"){
	var body : Rigidbody = hit.collider.attachedRigidbody;
	//als het geen rigidbody is doe niets
	if (body == null || body.isKinematic){
	//  als je onder duwt doe niets
	if(hit.moveDirection.y < -0.3){
	//push box in richting. Niet naar boven of naar onder (y op 0)
	var pushDir = Vector3 (hit.moveDirection.x,0, hit.moveDirection.z);
	body.velocity = pushDir * push_box;


Setting velocity is good for launching something, or some other 1-time event. It just kills any speed the object has, and sets it on an exact course. But, suppose your box was falling. Setting velocity with kill all the accumulated gravity, and the speed will appear to jerk.

Otherwise, you might try body.Addforce(pushDir*scaleFactor);. That will allow you to push the box faster and faster (if you can catch it,) for example.

Run some tests and see when and how it jerks (on the first hit? As you push into it? On flat ground? Rough ground?