I'm piecing a quilt with my wife's 319K. It was sewing OK fine, then the top tension went loosy goosy. I looked at the top tension closely and noticed a lot of oil. That is when I took a piece of folded fabric and cleaned between the disks. That has corrected the problem so far. But eventually the top tension will need to be removed, cleaned and readjusted.
Oil that you can't see down between the disks can cause the thread to stick between them.
Joe