I wouldn't mess with the bobbin tension. When you rethreaded, did you have the presser foot up? If not do it again-when the foot is down the tension thing is different so the thread doesn't go in right. Make sure it's fully engaged in the tension discs. Not all the way up like you do when you're engaging the accufeed foot but in the normal up position. Or try a new needle, maybe. I would probably just use mode 1, I believe stitch 11? I can't remember which is the zig zag. Now that I think of it, I haven't used that stitch on my machine yet... I don't think I have anyway.