Random loop rigidbody?

User presses key, force acts on rigidbody, rigidbody stops. User presses other key, other force acts on rigidbody, rigidbody stops.

 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'm trying to make the first force (transform.up) occur in a loop, and at random times within a set range.

I have tried:

InvokeRepeating Invokes the method methodName in time seconds, then repeatedly every repeatRate seconds. A coroutine is a function that can suspend its execution (yield) until the given given YieldInstruction finishes.

and several other methods.

The code as I have it now does not allow me to use the DownArrow command. I am finding it difficult to work with two different commands in the same FixedUpdate.

I appreciate any advice. Thanks.