Jump of 2d character is sometimes stronger.

Hey Community,

I have a problem setting up a 2D character. When I play it in the Editor my character works fine, but sometimes the character is jumping a lot higher then it should. Can somebody help me? Here is my code: (Yes, I want that the character is moving itself, without Input)

 using UnityEngine;
 using System.Collections;
 
 public class player : MonoBehaviour {
 
     public float speed;
     public float jumpforce;
 
     bool isgrounded;
 
     // Update is called once per frame
     void Update () {
     
         //Constant Force
         if (isgrounded) {
                         rigidbody2D.AddForce (new Vector2 (speed, 0));
                         rigidbody2D.velocity = Vector2.ClampMagnitude (rigidbody2D.velocity, speed);
                 }
 
         //Jump
         if (Input.GetKeyDown (KeyCode.Space) && isgrounded) {
             rigidbody2D.AddForce(new Vector2(0, jumpforce));
         }
 
         //reset
         if(Input.GetKeyDown("r"))
             Application.LoadLevel(0);
     }
 
     // check collision and update isgrounded
     void OnCollisionStay2D(Collision2D col)
     {
         isgrounded = true;
     }
 
     void OnCollisionExit2D(Collision2D col)
     {
         isgrounded = false;
     }
 
 }

Thanks for every answer and excuse my bad english ;)

Julian