Focus on all parts involved from stitch length lever to feed dogs, keep on cleaning, oiling, and make sure all parts move freely and nothing is stuck. If I remember correctly, the stitch legth lever moves in a sort of squarish groove behing there. You can carefully take out the two screws holding the cover behind the stitch leght lever, maybe you can detect something holding a torch light or reading lamp at a good angle. Poke in all the small oil holes with a tooth pick too, to make sure they aren't clogged up with dust. This should be pretty easy since your machine is clean and in other wise good condition.
Have the throat plate and bobbin cover off, poke around the feed dog area and under there with a tooth pick. Sometimes a piece of thread or tip of broken needle can hault things. Inspect behind the squarish cover in the back too and see if you can detect anything. Some oil points can take a bit of effort and repeated applications to reach. I tend to go over a machine from one and to another, cleaning, poking, turning the hand wheel by hand, inspecting moving parts, joints, hinges, gears,... Sooner or later you will spot the trouble area.
Best of luck, the only tricky thing with this model is as far as I know the needle size.
Last edited by Mickey2; 07-15-2020 at 05:25 AM.