My Pfaff did that for years. I would get the machine serviced and the foot control cleaned but sooner or later it would sew on its own (very laboured). I bought a new foot control but it did the same thing. Technician said it was one of the electronic components (this machine was top of the line in 1975) and at $50/hour might be rather expensive to find. I ended up buying a very nice Janome - but I still would take my old Pfaff back in a minute if I could get it fixed.