Are you freemotion quilting or straight stitching? I have the same machine and have never had to adjust the tension when quilting. When I have problems like you've mentioned it's either because something isn't threaded quite right or because there's a build up of lint and crud under the bobbin case. Try rethreading, and take the throat plate off, remove the bobbin case and clean out anything that's built up under there. It's amazing what a difference that can make! Good Luck!