A video cam is not ideally suited for stop-motion. You’ll get way, way better results using a digital still camera. Better resolution and much better suited to frame by frame work. Even the cheapest digital still would be good for this.
To get back to your question, if you are experiencing back focus trouble with your camera it might have internal damage. I’d suggest sending it in for repair and calibration as this is not something you can fix on your own.
Noah