Spawn power ups after random amount of time

So at the moment I have a power up that speeds the player up by 2x, in IEnumetor WaitTime ()

This is called when the Player gameobject hits the speed power up gameobject. I was wondering once it is hit, how to make it destroy and instantiate after a random amount of time?

Many thanks

Code:

 public float speed;
 private Rigidbody2D rb2d;
 public GameObject Player;
 public static int livess = 3;
 public Vector2 movement;

 void Start ()
 {
     rb2d = GetComponent<Rigidbody2D> ();
 }
 void FixedUpdate () 
 {
     float moveHorizontal = Input.GetAxis ("Horizontal"); //Gets keys that unity refers to being able to move horizontal (Set by default)
     float moveVertical = Input.GetAxis ("Vertical"); //Gets keys that unity refers to being able to move vertical (Set by default) Use forces to act with RigidBody2D
     Vector2 movement = new Vector2 (moveHorizontal, moveVertical);
     rb2d.AddForce (movement * speed);
     if(moveHorizontal < 0)
         GetComponent<SpriteRenderer>().flipX = true;
     else if(moveHorizontal > 0)
         GetComponent<SpriteRenderer>().flipX = false;
 }
 void OnTriggerEnter2D(Collider2D other)
 {
     if (other.gameObject.CompareTag ("shuriken")) 
     {
         if (livess <= 1) 
         {
             Destroy (Player);
             livess = livess - 1;
             Debug.LogError (livess);
         } 
         else 
         {
             livess = livess - 1;
             Debug.LogError (livess);
             return;
         }
     } 
     else if (other.gameObject.CompareTag ("speedPowerUp"))
         StartCoroutine (WaitTime());
 }
 IEnumerator WaitTime ()
 {
     speed = speed * 2;
     yield return new WaitForSeconds (5);
     speed = speed / 2;
 }

}