It would be a good idea to buy a Mityvac so you can actually see how much vacuum you're pulling. You may also have an injection problem - leaky fuel distributor, bad injector, or regulator, or etc.

You may have a bad EGR valve or it is opening at the wrong time - that will induce a massive vacuum leak. Do a search - lots of posts on how to test for that.

You can see the effect of the cabin vacuum leak by removing the connection on the back of the intake manifold, and plugging it. You should carefully examine all the rubber connectors and plastic tubes on the engine, especially the ones that get hot. Spray some engine cleaner or penetrating oil (use a spray nozzle with a tube so you're precise) around the intake manifold gaskets and injectors, and see if the idle changes momentarily.

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Chuck Taylor
Falls Church VA
'66 200, '66 230SL, '96 SL500. Sold: '81 380SL, '86 300E, '72 250C, '95 C220, 3 '84 280SL's '90 420SEL, '72 280SE, '73 280C, '78 280SE, '70 280SL, '77 450SL, '85 380SL, '87 560SL, '85 380SL, '72 350SL, '96 S500 Coupe