I do a lot of free motion and when my stitches look like that it means I forgot to up the tension on my upper thread. The upper thread tension has to be tight enough to pull the bobbin thread up halfway through the fabric. When I was learning about tension I was told to never mess with the bobbin tension, but to use the upper thread tension. Tighten it to pull up the bobbin thread, loosen it to let the bobbin thread pull the upper thread.