This thread is really interesting to be because I am seriously considering the purchase of a 1984 500SEL. I will know for sure when I look at it on Friday afternoon. The reason why it is so interesting is because I was also considering a 1985 500SEL that has a very similar problem that you are describing your 84' has. Hopefully this is not a trend on the 84-85 500SEL. I know that problems vary from car to car, but these are two of that I know of with the problem and it is not like these cars are all over. Luckily, as far as I know, the 84' I am considering is supposed to be in perfect running order, so hopefully that is really the case and it will stay that way for a while.

Good luck to you and keep us posted.