There are other places where you can have a vacum leak.
We have a different engine with a different fuel injection system, but I had a fluctuating idle, and after replacing a lot of parts, the problem turned out to be a leak at the intake manifold gasket.
Two independent mechanics that specialize in Mercedes failed to find the leak, but the dealer found it in less than an hour.
The moral of the story is:
If you throw enough money at a problem, you are eventually going to fix it.
2001 E430, Bourdeaux Red, Oyster interior.
1973 280SE 4.5, 170,000 miles. 568 Signal Red, Black MB Tex. "The Red Baron".