I recently bougth a 301 on Ebay and it had the same problem even after I cleaned her up really good. I remember how my sewing teacher in grade school (dating myself I am sure) taught me to check the bobbin tension. I put the bobbin in the bobbin case and just held the whole thing up by the thread and let the bobbin case and bobbin drop. Found that the bobbin case was not tight at all and the whole thing fell too fast. Used a screw driver to tighten the bobbin tension and tested again. She was much slower falling that time. So I threaded her up and when I started sewing, she sewed like a dream. Hope this helps.