I went thru the same thing. I cleaned and adjusted and cleaned and adjusted, changed needles, rethreaded, took bobbin out, put bobbin back in and was sooooo frustrated. I was telling my friend about it at quilt guild and she mentioned that the last time she had a problem like that it was the thread. I decided I had tried everything else so I would try a new spool of thread. I went and bought one wound a bobbin, rethreaded and finished FMQing the quilt in 20 minutes! That might be your problem also.