I had troubles with my tensions.....I agree to all of the above, especially the winding the quilt sandwich too tight.
Another thought....the bobbin case --- the case the bobbin goes into --- if you drop it, or it has a manuf. problem, it WILL cause you tons of grief. Sometimes off and on, sometimes ALL the time. Try buying a new bobbin case. Heck the spring in it may just have worn out!
My machine requires me to change the tensions with EVERY different type of thread. If I use Superior threads, I have to work at the tension with EVERY bobbin change (GRRRR). I now use MetroEmb. thread on my machine and have none of the problems. EXCEPT I have a habit of tightening the quilt too tight! If I start having problems, I drop the tension on the quilt back 2 notches....then all is fine.
