How are your brakes? Sometimes a leak through the master cylinder and booster diaphragm can make its way into the vacuum boost tube and produce similar results. The first things I would do are compression test and then vacuum test using a Mityvac on the brake circuit if the compression checks out OK. Of course if you're not losing any brake fluid then you can probably rule out the brakes.

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08 W251 R350
97 W210 E320
91 W124 300E
86 W126 560SEL
85 W126 380SE Silver
85 W126 380SE Cranberry
79 W123 250
78 W123 280E
75 W114 280