When I initially got my 1985 300D 2 years and 45k miles ago, I had the same problems. I traced them to be caused by several connectors and three of the five vacuum servos behind the dash panel which I have perfected the skill of replacing in record time (3 minutes - I timed myself) These servos cost about 35 each and are another common thing.
Without knowing the history of the car, I would start by making a general assessment of all the vacuum connectors in the engine compartment. Try to see if they are brittle or porous looking with little cracks in them or swollen at the end where they connect to whatever it is. Also, take a look at the main vacuum supply line going to the brake booster and clean out any ports near the booster on this tube as they often will clog. You can use a needle to get in the small orifice.
Put a vacuum guage in many areas throughout the system, especially at this main line and ensure you get about 15-20 psi vacuum. Pumps are cheap to rebuild - about 40 bucks - it might even be one of the ports/ valves in the pump that you could simply clean. I "biffed" my plunger shaft in the pump when rebulding mine, so be careful when tightening this nut that holds the pump piston in place if you rebuild. I bought a new one and it cost in the area of 170 or so.
Sometimes the vacuum system will decide to be lazy based on the perfect alignment of the stars. But really, the heat and temp extremes in this summers weather can cause things in the vacuum system not in top condition to work intermitantly.
Have fun with it,
Adam Bush