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