unity – Dynamic bridge via joints

When a new object is added to the bridge, the bridge shakes violently, and when objects are added even faster, it breaks.

I’ve tried changing mass and changing time step, but that doesn’t solve the problem.

enter image description here

Code for adding a new object:

(SerializeField)
Rigidbody _last;

(SerializeField)
GameObject _prefab;

void Update()
{
    if (Input.GetMouseButtonDown(0))
    {
        var go = Instantiate(_prefab);
        var fj = go.GetComponent<FixedJoint>();
        fj.autoConfigureConnectedAnchor = false;
        fj.connectedAnchor = new Vector3(0f, 0f, -2f);
        fj.connectedBody = _last;
        _last = go.GetComponent<Rigidbody>();
    }
}

The first cube is kinematic.
prefab:

enter image description here

How can you solve this?
Thanks for help