Car wheels 'flicker' when in freefall.

When my vehicle flies through the air (its a 4 wheeled car) after several seconds the wheels start acting oddly.
It appears as though they hit an invisible object, causing them to ‘push back’ into the suspension as it would were it hitting the ground. The chassis is unaffected as carries on its trajectory but the wheels just keep hitting this invisible object again and again very quickly, making it look as though the wheels are flickering.
Obviously there is no actual invisible object, it just appears that way. It happens in any part of my level, wherever the car freefalls.

They never did this until a week or so ago and now its doing my crust in!

Any idea where I should look to work this out? The wheel meshes are just for decoration, they are attached to wheel colliders.

I had a flickering problem and solved it by using WheelCollider.GetGroundHit instead of Physics.Raycast. The commented-out code caused flickering:

	// RaycastHit hit;
    WheelHit hit;
	Vector3 wheelPos;

    //if (Physics.Raycast(transform.position, -transform.up, out hit,wheelCollider.radius+wheelCollider.suspensionDistance) ){
    //    wheelPos = hit.point+transform.up * wheelCollider.radius;
    //    wheelTransform.position = wheelPos;
    //    airborne = false;
    //}
    if (wheelCollider.GetGroundHit(out hit))
    {
        // Code from http://answers.unity3d.com/questions/13180/using-wheel-colliders.html
        wheelPos = wheelTransform.localPosition;
        wheelPos.y -= Vector3.Dot(wheelTransform.position - hit.point, transform.up) - wheelCollider.radius;
        wheelTransform.localPosition = wheelPos;
        airborne = false;
    }
	else {
        // We're airborne. If we were already airborne in the previous frame, do nothing.
        if (!airborne)
        {
            wheelPos = transform.position - transform.up * wheelCollider.suspensionDistance;
            wheelTransform.position = wheelPos;
        }
        airborne = true;
	}