I think if the tension is appropriate when straight stitching, it might not be a tension problem. I think maybe you are moving your fabric faster than your needle. Does it happen when you FMQ a straight line or perhaps on a curve? When I took my FMQ class, I had this happen alot, especially when I changed directions quickly. Also, the instructor told our class that once you drop your feed dogs, there isn't any tension on the threads, top or bottom. . .