I never fully investigated the problem, but I believe this is caused by the follow-camera always attempting to move behind the car, and the car velocity changing rapidly from positive to negative and vice-versa.
There is also a bug in Car.js which I think exacerbates this problem (the car accelerates when the throttle isn't being pressed). I believe the logic there should be something like this instead:
if (throttle == 0) {
// Mathf.Sign returns 1 when a value is 0, (also happens in HaveTheSameSign)
// this causes inappropriate acceleration unless we check whether throttle is 0 first... :-/
} else if (HaveTheSameSign(relativeVelocity.z, throttle)) {
if (!handbrake) {
throttleForce = Mathf.Sign(throttle) * currentEnginePower * rigidbody.mass;
}
}
else {
brakeForce = Mathf.Sign(throttle) * engineForceValues[0] * rigidbody.mass;
}
Not sure this will totally solve the problem, but it might help.