When similar happened to me in other cars, it is because the ignition key housing is rotating. That was in older 60's cars with a simple screw-in dash switch. Your 83 240D is likely the same as my 84 300D, with the key switch in an extension tube off the steering column. I took mine apart to fix a jamming key, due to a broken spring detent in the rear. As I recall, the key innards are held in by a locking wire spring that pops in a little hole in the housing. You must get the key in a certain position to push the wire free of the hole to pull the innards out. You can find instructions and photos here in a search.