how can my gameobject stop shaking?

so i'm making a 2d game, and my player (sphere with char. controller , rigidbody and that) and 1 monster (large cube) can move just in the x axis. so, when i jump over the cube, it rotates to follow the player, but in the process it shakes a lot, the large cube almost fall in one side, like if it were made of paper. what can i do? also, how can i make a solid gameobject? i mean, when the player and monster collide, the player flies away and the cube falls in the ground trying to stand up, what can i do to make a realistic collide? thanks!

using UnityEngine; using System.Collections;

public class mobAI : MonoBehaviour {

 public Transform target;
 public int movespeed = 4;
 public int rotationspeed = 4;

 Vector3 MoveVector;


 private Transform mytransform;

 

 
 void Awake()               //se agrega awake antes de todo, sera lo que pase primero
 {
     mytransform = transform;
 }


 void Start()
 {
     GameObject AI = GameObject.FindGameObjectWithTag("Player");
     target = AI.transform;
   
 }
 

 void Update()
 {


     Vector3 pos = transform.position;
     pos.z = 0;
     transform.position = pos;
   
   
 

     Debug.DrawLine(target.position, mytransform.position, Color.blue);
         //---mirar al target---
         mytransform.rotation = Quaternion.Slerp(mytransform.rotation, Quaternion.LookRotation(target.position - mytransform.position),rotationspeed * Time.deltaTime);
     
     //toma la rotation de transform, y la gira del punto A (mytransform.rotation) al punto B (Quaternion.LookRotation), adentro del parentesis se indican los terminos
     //para girar : hacia: target.position y se resta la propia rotacion actual
     //*deltatime lo convierte a un movimiento en tiempo real dependiendo de los FPS de cada PC
   
         float distance = Vector3.Distance(target.transform.position, mytransform.position);
             //---mover hacia el target---
     
         if (distance > 8.1f)
     {
         movespeed = 0;
     }
     if (distance < 8f)
     {
         movespeed = 4;
     }
     if (distance < 1.9f)
     {
         movespeed = 0;
     }
     
 
 
     mytransform.position += mytransform.forward * movespeed * Time.deltaTime;
   }
         
     

}