In The Same Boat...
I have the same problem on my '82 300SD! I was told that there is a leak somewhere in the vacuum system that controls the doors, trunk, and fuel door locks. I checked the vacuum/pressure pump (it's in the trunk to the right rear of the spare), and it works fine. The leak is somewhere in the tubing that makes it all work, or in the doors where the vacuum meets the actuators. Bottom line. Are you savvy enough to diagnose it on you own? Do you have a presure/vacuum pump with a guage on it? Does it really mean that much to you? Can you afford to have a local shop diagnose and fix it for you?
These are the questions you need to ask yourself, along with the "Why didn't I already search the archives for an answer?" question. I haven't found it to be such an issue that I have actually done an archive search before now, but now that you have reminded me...
"We drive into the future using only our rearview mirror."
- Marshall McLuhan -
Northern California Wine Country...
"Turbos whistle, grapes wine..."