I had a problem similar to this, kept adjusting the tension.... nothing helped. One day my daugher stopped my, asked if I had cleaned out the bobbin area lately - huh? "No", I sheeplishly replied.... she cleaned it and everything worked like a charm again. I had so much stuff packed in around the bobbin holder, it look like a piece of felt! If you haven't tried this, I suggest this as a possible solution - good luck :)