Question rigid body jumping in place

I make a controller on a rigid body and stopped at the next moment, the object jumps in place constantly, here is an example

Code (CSharp):

1.  
2. if(!Ground){ MoveDir.y += -1f; }else{ MoveDir.y = 0f; }
3. if(MoveDir != Vector3.zero){ // MoveDir is Vector3 x, z axis vertical and horizontal
4.     RB.MovePosition(RB.transform.position + MoveDir * 4f * Time.fixedDeltaTime);
5.     }

that is, as planned, it should go down if there is no ground under its feet, while the ground is checked by a sphere under the collider (0.2 lower).

I don't understand why he jumps in place...