Fullscreen FPS the right way?

FPS mousemove2.as

stage.addEventListener(MouseEvent.MOUSE_MOVE, mousemove, false, 0, true);
scene3D.addEventListener(Scene3D.UPDATE_EVENT, update, false, 0, true);

private function mousemove(event:MouseEvent):void
{
    if (event.movementX != 0) {
        const mouseMoveX:Number = (event.movementX);
        spinX += mouseMoveX * 0.035;
    }

    if (event.movementY != 0) {
        const mouseMoveY:Number = (event.movementY);
        spinY += mouseMoveY * 0.035;
    }
}

private function update(event:Event):void
{
    player.rotateY(spinX, true);
    spinX *= .51;
    
    scene.camera.rotateX(spinY, true);
    spinY *= .51;
    
}

When I move the mouse left slowly while moving forward (WASD), the camera goes spastic. That's why I tried to fix it with spinX *= .51;

Still not great.

Any help appreciated :)