Rigidbody is bugging

Hello,
Since i reopened my project today everything is acting weird, i might have changed something before i tested it.
So it could be my code too, but it's doing this:

basically, once i press spacebar it gives the player a small push which drives him forwards, building momentum.
that did work well untill now, from 0:10 seconds on i stop pressing the spacebar, and it just keeps on going with the same speed, it doesn't increase or decrease, and all of a sudden, it jumps to a totally other speed!

What could be the cause of this?

this is what makes it move:

Code (CSharp):

1.     void FixedUpdate () {
2.        OpDeGrondBlijven();
3.        // float moveHorizontal = Input.GetAxis("Horizontal");
4.         float moveVertical = Input.GetAxis("Vertical");
5.  
6.         Vector2 movement = new Vector2(1f, 0.0f);
7.         if (Input.GetKeyDown(KeyCode.Space) && Energy.value >= 1)
8.         {
9.             Energy.value = Energy.value - EnergyWaarde;
10.             //if (SliderScript.SliderValue > 46.5 && SliderScript.SliderValue < 54.6)
11.             // {
12.             rb.AddForce ( movement * SliderScript.SliderValue * StrengthAmount);
13.             //rb.AddTorque ( 1 * SliderScript.SliderValue * StrengthAmount);
14.             //rb.velocity = new Vector3(10, 0, 0);
15.             //}
16.  
17.         }
18.  
19.  
20.     }

Greetings,
Timo

 

BUMP, i still haven't managed to solve it.