How to rotate smoothly

Hey, I started Unity a week ago and until now some search here help me but now I'm kinda stuck. The thing is, I want to rotate an arm smoothly with the mouse Y coordinate so I try to use Quaternion.Slerp and all but the arm still teleport from one position to another if, for example, I let go the mouse button and I click on another Y position. Here is my code:

     void Start () {
         ArmStart = transform.rotation;
     }
     void Update(){
         Vector3 mousePos = new Vector3 (Input.mousePosition.x, Input.mousePosition.y, 10);
         Vector3 lookPos = Camera.main.ScreenToWorldPoint (mousePos);
         Vector3 BoundPos = Camera.main.ScreenToViewportPoint (mousePos)*100;
 
         if (Input.GetMouseButton(0)&& BoundPos.x < 96 && BoundPos.x > 65 && BoundPos.y < 90 && BoundPos.y > 55) {
             float speed=50f;
             float angle = ((Mathf.Atan2 (lookPos.z, -lookPos.y) * Mathf.Rad2Deg) - 130) * 2.8f;
             Quaternion ArmGoal = Quaternion.AngleAxis (angle, Vector3.forward);
             transform.rotation= Quaternion.Slerp(ArmStart,ArmGoal,speed*Time.deltaTime);
             ArmGoal=ArmStart;
         }
     }

Don't mind the condition for the if, it's cause I want that we can control the arm inside precise bound. If you have any idea why it don't work or if you have a better idea... or if you just want to laugh at a damn noob answer please.