Maybe someone else has had a similar problem with a 15, the closest I have had is packed up dust under the throat plate, it sort of gives similar problems with stitch length and lever. Regardless, since it's a newly required machine I would keep on oiling every joint, hinge and gear; if you are lucky the trouble might be caused by a stuck part some where. I would pay particular attention to every link from stitch length lever to feed dogs; repetetive oiling, and investigating under good ligh conditoins, like torch or reading lamp.
A dry machine can need quite a bit of oiling to run well again, I have had to keep oiling for up to four days to free up a machine and it was just a bit sluggish to start with. You might need to get the hand wheel off to get to the grease parts behind the hand wheel, there is a worm gear that needs to be checked. Poke a tooth pick down the holes on top, and make sure you detect all the oil points, it's easy to miss a few. Every part where metal moves against metal needs lubrication.
Last edited by Mickey2; 06-07-2020 at 03:10 PM.