The old Pfaffs freeze up because they have a thick lump of grease on top of a set of gears. It provided lasting lubrication, but not after it turned sticky. If this isn't somehow dissolved, flushed out or properly cleaned out they will stick. The 401 is a bit different this way, maybe some of the joints or parts manage to gather oil in large enough quantities for it to stick as it dries out. I'm not sure, either way, any machine seem to be able to stick or freeze up. There is something with the 401s and related models bkay, because they often need much more tentative work than other zigzaggers to unstick.