Jumping through the roof!

Hello,

I've made the follwing jump code. Time is frozen and the character is stuck mid air. When you click on the screen force is applied so he jumps for a bit.

But once every 10-15 times, he goes through the roof when i first click the screen. I have no idea why this is happening or how to fix it. i've tried a lot of different things but since i have no idea what the problem is i cannot fix it. Help would be appreciated.

You can see for yourselves in a version i made for kongregate ( press space to jump)

http://www.kongregate.com/games/axell/bouncy-ninja

 using UnityEngine;
 using System.Collections;
 
 [System.Serializable]
 public class JumpNinja : MonoBehaviour {
 
 
     // The fx,fy,fz - forces in x/y/z direction. by default, fz = 0 
 
     public float fx;
     public float fy;
     public float fz;
     public GUIText tapText;
 
     void Awake(){
         Time.timeScale=0;
         Debug.Log("timescale");
         //rigidbody.AddForce(fx,fy,0);
         }
 
     void Start () {
         rigidbody.AddForce(fx,fy,0);
     }
     
     void Update(){
     
         if (Input.GetMouseButtonDown(0))
         {
             Debug.Log("Working");
             if (Time.timeScale == 0)
             {    
                 tapText.guiText.enabled = false;
                 Time.timeScale=1;
             }
             audio.Play();
             rigidbody.AddForce(0,fy,0);
             rigidbody.velocity = new Vector3(rigidbody.velocity.x, 0, rigidbody.velocity.z);
             //rigidbody.AddForce(0,fy,0);
         }
     }
 
 }