I think QuiltnNan may have the answer. Tiny threads that get caught under the spring in the tension can cause havoc. Aargh, it's early morning for me and explaining anything is rough. Anyway, the idea is to gently insert a fine pin under the tension spring located in the bobbin case, sliding it to the end of te spring, gently checking for/removeing any threads or thread lint.
Somewhere on the Board is a wonderful explanation of how to check that your bobbin tension is right. Forgive me but I do not have the specific site at the tip of my tongue and today's project is an early leave time for a road trip to central FL. I'll find it when I get back if I don't see it on today's board.
Pat