I was having that problem with my Singer, but finally figured out that I was the culprit. Many times during sewing, I will get up to do something. Often, as I walked away, I would hear/see the machine stitching away. Took me awhile to figure out that when I stood up, I would unconsciously move the foot pedal barely under the platform base of the table I sew at, which is elevated 3 inches off the floor. Walking and moving around would vibrate the floor/table and since the pedal was wedged under the base, it would sew on its own.