I have contacted Singer for the correct bobbin size, but it still bounces up and down while sewing. It gets really bad after I have only used about half of the bobbin. It messes up the tension, so I start out looking good and the more I sew, the worse it looks. Adjusting the tension has not helped. I have tried Gutterman thread, Coats and Clark, even serger thread to no avail. Any ideas?