Please help me stop the flipping card!!!!!!!!!!

I am seriously going to explode... I have tried about 6 different ways to get a card to flip 180 degrees to show the other side but every single time the card just keeps on going. it never stops and i cannot figure out why. here is my current itteration. i have also tried to animate it but that was having the same problem, and i hate the new severe reliance on the animation controller; but i digress. please help!!!!

here is the flip section

     void Update () 
         {
             if(_flip == true)
             {
                 targetRotation = Quaternion.AngleAxis(180.0f, transform.forward) * transform.rotation;
             }
             transform.rotation = Quaternion.Slerp(transform.rotation, targetRotation, smooth * Time.deltaTime);
         }

here is the full code

 using UnityEngine;
 using System.Collections;
 using System;
 
 public class Flip : MonoBehaviour 
 {
     //setup variables that are needed in script.
     private Vector3 mousePos;
     private GameObject _ts;
     private Quaternion targetRotation;
     private float smooth;
     public GameObject obj;
     private GameObject _obj;
     public GameObject selectionMasterScript;
     private bool _flip;
     private Quaternion _rotation;
     //public Animator animator;
 
     // Use this for initialization
     void Start () 
     {
         _ts = this.gameObject;
         _rotation = this.gameObject.transform.rotation;
         _flip = false;
         //animator = GetComponent<Animator>();
         selectionMasterScript = GameObject.FindGameObjectWithTag("SelectionMaster");
         //declare variables starting condition.
         targetRotation = transform.rotation;
         smooth = 3.0f;
         obj = null;
         _obj = selectionMasterScript.GetComponent<SelectionMasterScript>().thing;
     }
     
     // Update is called once per frame
     void Update () 
     {
         if(_flip == true)
         {
             targetRotation = Quaternion.AngleAxis(180.0f, transform.forward) * transform.rotation;
         }
         transform.rotation = Quaternion.Slerp(transform.rotation, targetRotation, smooth * Time.deltaTime);
     }
     
     void OnMouseEnter () 
     {
         //obj is equal to what ever you are hovering over.
         _obj = GetComponent<Collider> ().gameObject;
         selectionMasterScript.GetComponent<SelectionMasterScript> ().thing = _obj;
         //Debug.Log (_obj);
         //Debug.Log ("enter");
     }
 
     void OnMouseOver()
     {
         //Debug.Log ("hovering");
         if (Input.GetMouseButtonDown (0))
         {
             //animator.SetBool("Clicked", true);
             _flip = true;
             Debug.Log("Clicked");
         }
     }
 
     void OnMouseExit()
     {
         _obj = null;
         //Debug.Log (_obj);
         //Debug.Log ("exit");
         //Debug.Log (selected);
     }
 }