Prefab clones with same script and different random number?

I’m instantiating several Prefabs of the same kind. These prefabs should be moving up and down. To make the animation I’m using iTween.

void Start () {
		System.Random random = new System.Random();
		int randomNumber = random.Next(0, 99);
		float delay = (float) randomNumber / 100;
		iTween.MoveBy(gameObject,iTween.Hash("y", 2,"time", 1, "looptype", "pingpong", "delay", 0));	
	}

It seems every clone of the Prefab is using exactly this script (Start only called once). What would the correct way to do this be?

This works for me:
Declare the System.Random and the int outside of the Start(), then assign them in the start, so each time the script is started a new random number will be made…

public class YourClass: MonoBehaviour
{
System.Random rdn;
private int _tempRnd;

void Start()
{
    rdn = new System.Random();
    _tempRnd= rdn.Next(0,99);
    Debug.Log(_tempRnd);
} 

}