I have had problems with this in the past. I too can say that putting in a new needle and using a better quality thread made all the difference in the world. I use threads from my Bernina dealer. King Tut quilting thread I find is the best. I have also bought thread from Connecting Thread. Theirs is good stuff too. It just always seems that I need or want my thread now and that means a trip to my B-dealer. How ever if I plan ahead I have no problem with Connecting Threads.
I have made it a habit to use a new needle for every project. If I am working on a big project I may even put in a new one during. I would rather be safe then sorry.
Hope you have gotten to the bottom of your problem. And happy quilting.
Karol