Rigidbody jerky movement?

Hello all, first-time unity user and poster.

I'm working on a basic control system to fly a 3rd person spaceship around in 3D space using physics. I have a script which simply gets user input for positive or negative thrust, then multiples that by the direction and an acceleration multiplier. There's a bunch of other code, but slowly debugging by turning things off showed that neither mouselook aiming nor top speed limiting changed anything.

> forceToAdd = Vector3.forward *
> CheckForThrust()*accelMult;   
>   rb.AddRelativeForce (forceToAdd);

Checkforthrust:

>  function CheckForThrust(){
>       // If accelerate key is pressed, set accel to increase to 1. if
> backwards, interpolate to -1. If
> nothing, head to 0
>       
>       if (Input.GetKey("q")) {
>           engineStatus = Mathf.Lerp(engineStatus,1,0.01);
>       } else if (Input.GetKey("e")) {
>           engineStatus = Mathf.Lerp(engineStatus,-1,0.1);
>       } else {
>           engineStatus = Mathf.Lerp(engineStatus,0,0.01);
>       }
>     
>       Mathf.Clamp(engineStatus,-1,1);
>       return engineStatus;
>     }

When acceleration starts, everything's fine. But when I get past a certain velocity threshold the ship starts jittering like mad. It's not related to application of force, because if I ease up on the thrust and stop accelerating, the jittering remains. I tried setting the rigidbody to Interpolate and Extrapolate. Extrapolate seemed to improve things a bit, but not much. I even tried applying the force inside Update() instead of FixedUpdate() even though you're not supposed to, that just made it worse.

Any ideas?