It could be a million things - do a search. Providing more background info would be good too. On the W126, common causes of rough idle are dirty ICV, faulty ICM, leaky cold start injector, vacuum leak, worn secondary ignition components, faulty O2 sensor, etc. etc. Again, 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