Seems like your car has a vacuum leak which increases the amount of air in the air/fuel mixture or your car's fuel distibutor is adjusted to supply the lean fuel.
Last week I was having the same problem on my W124 230E. It turned out the the hose coming from the idle control valve was deattached.
Good luck.

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1989 230E, 8v, 166.000 km, updated to 94/95 trunk & hood
2002 Daewoo Nexia 50.000 km
Sold:
1987 VW Jetta GLE 16V, Recaro seats
1982 Volvo 240 DL (lovely car!)
and few more american cars.