Random Y Axis Direction On Prefab Within Boundaries (C#)

Hi guys, I've been having this issue for ages and none of my Google searches have turned up anything decent that helps me.

So basically I have a script that generates instances of 5 prefabs every second, in that those prefabs have a script that tells them to move left on the x axis (2D game), now what I need is for that script to give each instance of each prefab a random direction on the y axis to travel (up or down obviously), but it has to be different each time.

I also need to constrain it somehow, so it doesn't go out of a certain range, and when it hits the edge of that range, I need it to change direction to the opposite direction on the y axis.

Here is what I've most recently tried:

 using UnityEngine;
 using System.Collections;
 
 public class PowerItem : MonoBehaviour {
 
     public float itemUp = 0.1f;
     public float itemLeft = 0.7f;
 
 
     public float x;
     float y;
     float z;
     Vector3 pos;
     
     void Update () {
 
         //transform.position = Vector2(Random.Range (1,4),Random.Range (1,4),10);
 
         transform.position -= Vector3.right * itemLeft * Time.deltaTime;
 
             x = 0;
             y = Random.Range(-100, 100);
             z = 0;
             pos = new Vector3(-x, y, z);
             transform.position = pos * itemUp * Time.deltaTime;
         }
     }

Is there anybody who would be kind enough to help me out here? I'm really stuck.