You car has a pneumatic governor and it probably has a ruptured diaphram.
To test remove the red air filter from the rear of the pump. Place your thumb over the opening while the engine is idling. If the engine speed is affected by this then the diaphram is bad. The reasoning here is the diaphram passes air which comes through the vent in the air filter. When you put your finger over the hole (sealing it of course)you remove this extra air which is passing through the diaphram. The diaphrams aren't too expensive and are a little tricky to replace but can be done by an experienced tech in less than an hour I'd say if there are no frozen bolts.
Owner 24 bay BSC
Bosch Master, ASE master L1
26 years MB technician