I feel for you, it's so discouraging when your machine won't cooperate. I had quite a similar problem with my Bernina 440 a short time ago. I took it to the dealer twice. For some reason it would not maintain the tension and I like you had entire lines of stitches that would out with a little pull. I knew it was the tension, but could not fix it myself. Iv'e have it back now since November and it seems to be working fine. I do find that spray basting causes some issues for me, but I still use it, cause I'm a lousy pinner! Do try a larger needle, and try the same weight thread in top and bobbin.