Object with rigibody bouncing all over the terrain when moving

I have a terrain that's about flat but has some light bumps. I have an object with a rigibody that I move using a basic script:

         float forwardThrust = Input.GetAxis("Vertical") * thrust;
         rb.AddForce(transform.forward * forwardThrust);

         float turningTorque = Input.GetAxis("Horizontal") * torque;
         rb.AddTorque(transform.up * turningTorque);

It works ok for the most part, but one issue is as it picks up even reasonable speed it starts bouncing all over the place uncontrollably. I set the weight to 1000, and physics materials on both the terrain and the object have bounce=0. I can't figure out what I'm missing. I want the object to glide along on the terrain and quit bouncing.