Crown Point SZA-645F sewing machine jams?

My Crown Point SZA-645F suddenly started to jam at the same point on every full turn. Initially I could move it by turning the hand wheel by hand, then it became impossible. Obviously I don't want to burn the motor out, I've looked inside and cleaned / oiled everything that moves but can't see what is causing the freeze. It appears to be connected to the stop motion knob in the hand wheel, but not sure – any ideas please?