I have the same machine, and yes it appears to be due to poor bobbin winding. I found my bobbin doesn't make good contact with the spring at the top of the spindle when winding so it spins a little unevenly. I put a small washer under the bobbin to raise it and it's fine. Tried felt but the lint snagged in the bobbin thread. Until I figured it out I used a separate bobbin winder, and no more problems. Also- the machine is Very sensitive to lint, so remove the entire cover plate and bobbin holder and clean behind and underneath OFTEN. Be Sure to put a mark on the front of the bobbin holder before removal so you know the exact positioning to replace it. It's not clear in the manual. I have a lot of newish Coats and Clark thread to use up before I start buying anything else.