My gut reaction is also that it's the top thread. To verify, put different colors in the top and bobbin. When this happens, don't move the fabric immediately. Stop sewing and verify that the top thread is still completely threaded correctly. Still in take up lever? still in the tension discs? Normally, you thread with the presser foot up so that there is no tension in the discs, then when you get to the needle, you put the presser foot down to seat the thread. It's always a good idea to get out your manual and make sure that you are threading correctly. When you start to sew, pull up your bobbin thread and hold both tails as you take your first stitches. Clean your machine & floss between the discs with a sturdy thread, and check that there's no lint in the tension spring in your bobbin case.