Jumping is to high sometimes (inconsistent)

Hey, Im currently having the problem that my Sphere jumps inconsistent sometimes. Sometimes it jumps on the height i set to it and some and jumping way to high. Any ideas to fix it.

using UnityEngine;

using System.Collections;

public class PlayerController : MonoBehaviour

{

    public float speed;

    private Rigidbody rb;

    public LayerMask groundLayers;

    public float jumpForce = 7;

    public SphereCollider col;

    void Start()

    {

        rb = GetComponent<Rigidbody>();

        col = GetComponent<SphereCollider>();

    }

    void FixedUpdate()

    {

        float moveHorizontal = Input.GetAxis("Horizontal");

        float moveVertical = Input.GetAxis("Vertical");

        Vector3 movement = new Vector3(moveHorizontal, 0.0f, moveVertical);

        rb.AddForce(movement * speed);

        if (IsGrounded() && Input.GetKeyDown(KeyCode.Space))

        {

            rb.AddForce(Vector3.up * jumpForce, ForceMode.Impulse);

        }

    }

    private bool IsGrounded()

    {

        return Physics.CheckCapsule(col.bounds.center, new Vector3(col.bounds.center.x,

            col.bounds.min.y, col.bounds.center.z), col.radius * .5f, groundLayers);

    }

}