So, here's my dilemma. I recently purchased a 15-91 which was in almost perfect shape in a cabinet. I took it to a sewing 'mechanic' to give it the once over, including 'lube and oil'. He replaced the old bobbin case stating it was worn and wasn't getting good tension. Once I got it home I had to adjust the upper thread tension slightly. I was able to piece a small quilt but it became apparent there were hidden issues when I began free-motion quilting. At times the upper thread would get caught around the bobbin case. I've taken it back 3 times to have it adjusted or to see if the experts could figure out the problem. They checked for burrs and couldn't find any. The final thing they did was to replace the tension spring. What is really puzzling me, it doesn't seem to happen on a consistent basis. I've changed the thread, needle, put in a larger needle, used different bobbins, threaded, rethreaded, adjusted the thread and bobbin tension. It seems to happen when stitching for slightly longer than piecing blocks and not only when free-motion quilting. I have my fingers crossed hoping someone here has a idea what it could be.