My '72 W200 does the same thing. On mine, if I turn it just until the starter cranks, it'll start. If you crank the switch hard against the 'start' position, it'll crank all day without starting. I tried to run it down once and the best I came up with was that the lock mechanism had worn so much that when you turn to 'start' hard, it allows the ignition switch to turn past the 'run' position.