I had this happen to me in an old 190d, and while this is not a long term cure, maybe it will help in the shortrun. Try pushing the key in all the way, then pull back very slighly(1/16 of an inch???)and turn. Hope this helps.

__________________
1987 300SDL
1990 300E
1989 300E
1986 300E
1981 300D
1983 240D