You said it sewed fine for quite a while, then all of a sudden it went bad. What changed at that time? Let yourself think about it for a while, maybe even tell yourself to dream about it when you go to bed. (Believe it or not, this has actually helped me solve some problems.)
I have a friend who was having some problems with her longarm, took them 2 years to figure out that she was at the end of the power line on her street. I don't remember why it was an issue, but she did get it resolved, much to her relief.