The machine is properly threaded and makes near perfect stitches through, both multiple layers of interfaced cotton fabric, and leather. Test swatches are about 9 inches long. After about four passes (straight stitching, then lifting presser foot and changing direction) the machine begins to malfunction. Stitches are no longer forming properly and the bobbin tension has gone haywire. After cutting the thread and removing the bobbin case, it is apparent that the thread has come unthreaded from the bobbin case.
We've cleaned and oiled the machine including removing and cleaning the shuttle driver and race to remove old thread dust and gummed oil. This problem started before removing the race. Attached is the test swatch top and underside.
Any thoughts on why the thread would come unthreaded from the bobbin?