I make sure I have the same weight in top and bottom - also rethread from start to finish including a different bobbin. Sometimes these mechanical marvels can stress a person to the limit. I was having some major problems with my Bernina 180 and finally took it in. The timing was off and I'd not sewed very much since a servicing. Go figure!! So as a last resort take it in to be serviced. Hope you have bood luck with all the tips.