I have an '83 Mercedee 240, and what I can say is that Mercedees is notorious for having vacuum seal leaks coming from where the seals are in the door locks. (they become brittle and let air in). so could be when you use the drivers side lock (which you have used the most), the seal is bad in that one.
If it is a vacuum problem and it is a diesel, eventualy you wouldn't be able to shut the car off. (or will start taking longer for it to shut off).
I usually tell people, the simple fix for THIS problem is to just plug the vacuum line located behind the firewall (next to the brake fluid container), you can just pull the vacuum line off and put a plastic bag over it with rubber band. (takes less than a minute to do). HOPE this helps for some of you.