Have object being displaced, want to randomize time of displacement within set parameters, then loop that, Unity C#

I have a transformation (force applied moves object) happening initially, and then the user responds to it with the DownArrow. I want that initial transform.up to happen at random times within a set interval, and for that to loop over and over, so that the user can continue to respond to it.

using UnityEngine;
using System.Collections;

public class thingBumps : MonoBehaviour {

public float thrust;
public Rigidbody rb;

void Start() {
    rb = GetComponent<Rigidbody>();
}

void FixedUpdate()
{
    rb.AddForce (transform.up * thrust, ForceMode.Impulse);
    if (Input.GetKey(KeyCode.DownArrow))
        rb.AddForce(-transform.up * thrust,ForceMode.Impulse);
}

}

I have tried Coroutines and also InvokeRepeating but I am having a very hard time and I have read every thread on the topic. I just want that first transform.up to repeat at random time intervals so the user can continue to respond to it (with their own -transform.up transformation). Does anyone know how to do this?

I appreciate any advice.