Are you 100% sure that you have it threaded correctly. If the thread is not over that little arm after the tension disks, then through the spring it can cause all sorts of weird issues. Others have mentioned the bobbin, but I seem to get issues with the thread not going over that little bar sometimes and it causes all sorts of messed up stitching. I would suggest setting the tension dial at between 3-4. This is the sweet spot for mine. Just a couple ideas.