can someone help me with the jumping,when i am pressing space to jump its not the same height everytime and my

public float jumpForce = 600;
public GameObject jumpCheck;
public LayerMask groundLayers;
public SphereCollider col;

col = GetComponent();

if (isGrounded() && Input.GetKeyDown(KeyCode.Space)) 
{ 
rb.AddForce(Vector3.up * jumpForce * Time.deltaTime, 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 * .9f, groundLayers);
    
}

Without seeing the code in full it hard to tell what’s going on where so I’m going to guess.
If the if statement is in the update method then it might be double jumping as depending at which point in the frame there may be time to jump again before leaving the ground check, this would be intermittent.
Set a bool for key press in Update and action in FixedUpdate and set boo back to false then can only jump once.

Also i use ForceMode.VelocityChange