The script is intended to rotate my Gun on the x angle when I move my mouse horizontal. My script works fine as long MoveOnX is positive, But when its negative, it changes the x, y and z rotation. Can someone help me?

GunMovement.js

public var MoveAmount : float = 1;

public var MoveSpeed : float = 2;

public  var GUN: GameObject;
public  var CAM: GameObject;


public var MoveOnX : float;

public var MoveOnY: float;

public var DefaultPos : Vector3;

public var DefaultRot : Vector3;

public var NewGunPos : Vector3;

public var NewGunRot : Vector3;

public var ONOff : boolean = false;

function Awake(){


  DefaultPos = transform.localPosition;
  DefaultRot = transform.localEulerAngles;
  ONOff = true;

}



function LateUpdate () {


   if(ONOff == true){

 MoveOnX = Input.GetAxis( "Mouse X") * Time.deltaTime * MoveAmount;
 MoveOnY = Input.GetAxis( "Mouse Y") * Time.deltaTime * MoveAmount;
 NewGunPos = new Vector3 ( DefaultPos.x+ MoveOnX, DefaultPos.y + MoveOnY, DefaultPos.z);
 NewGunRot = Vector3 ( DefaultRot.x + MoveOnX * 800, DefaultRot.y, DefaultRot.z);
 GUN.transform.localPosition = Vector3.Lerp(GUN.transform.localPosition , NewGunPos, MoveSpeed * Time.deltaTime);
 GUN.transform.localEulerAngles = Vector3.Lerp(GUN.transform.localEulerAngles , NewGunRot, MoveSpeed * Time.deltaTime);

}

else{

   ONOff = false;

 GUN.transform.localPosition = Vector3.Lerp(GUN.transform.localPosition, DefaultPos , MoveSpeed *Time.deltaTime);


}

}