I would start by looking for possible vacuum leaks, such as broken vacuum lines or ICV hoses (do a search). That "economy" gauge is actually a glorified vacuum gauge, and it sounds like you're only getting about half the vacuum you need. The gauge should only hit 50% when you are in mid-throttle or cruising. Too much air = too lean and won't run properly. Just my 2 cents, there are a hundred other things you could check on. Do a search.

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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