Clean it all out, CHANGE THE NEEDLE, rethread. I have a Janome embroidery machine and has had nothing but this kind of trouble. Sometimes I can fix it on my own and sometimes not. It has been in the shop 10-12 times for the same problem as you. Janome has a problem with the bobbin. It is noted in several blogs. Janome claims they have corrected the problem, but they haven't.