Rotation script issues?

This script is called when the user release the mouse button:

float rot_duration = 3f;
float rot_speed = 1.8f;
Quaternion final_rot;

void Start()
{
    cubeMesh = GameObject.FindWithTag("CubeMesh");

    Vector3 initial_rot = transform.rotation.eulerAngles;
    final_rot = Quaternion.Euler(new Vector3(initial_rot.x, initial_rot.y, 180));
}

public void Update()
{
    if (Input.GetMouseButtonUp(0)) 
    {
        StartCoroutine (DelayRotate (0.1F));
    }
}

IEnumerator DelayRotate(float waitTime)
{
        yield return new WaitForSeconds (waitTime);
        float rot_elapsedTime = 0.0F;
        while (rot_elapsedTime < rot_duration) {
        cubeMesh.transform.rotation = Quaternion.Slerp (transform.rotation, final_rot, rot_elapsedTime);
            rot_elapsedTime += Time.deltaTime * rot_speed;
            yield return null;
        }
}

This script makes a GameObject rotate, 0.1 seconds after mouse button release. The problem is that it "flips" the GameObject quickly then starts rotating.

I believe it is flipping due to final_rot2 = Quaternion.Euler(new Vector3(initial_rot.x, initial_rot.y, 180)); (because of 180 value) What should I do instead?