My 300E has been doing this since I bought the car. Cold and hot starts are fine. Intermittent problem. I'm getting 10 mpg in the city! But I won't throw dollars at it and paying a mechanic to trouble shoot it when I (with the help of fellow forum members here) can do most of it.
It seems that we should collect the number and exact location of all the vacuum hoses that seem to cause leaks and post them here. At least searchers in the future will have a good reference point.
For instance, I did not know about a hose to the brake booster. I'll be looking for it now.
For 1990 300E, I know of four hoses right near the air cleaner:
1) Breather hose from valve cover to air cleaner box
2) Vacuum hose from valve cover to manifold.
3) Two hoses connected to the Idle Control Valve.
Additional lines and hoses:
A hose from the manifold to the brake booster.
A small diameter hose from the air cleaner breather hose to the fuel pressure regulator. (Still need some help of jury rigging a new piece coming out of the fuel pressure regulator.)
Please add any more to the list.
Also, small diameter vacuum lines and their connectors are good ones to replace at this age. Any critical ones to start with.
This stuff is cheap and makes such a difference in our car's running performance.