Over here we call this an idle "surging" problem...
...whereas, the idle speed seems to fluctuate in some cyclical fashion when stopped.
I have been trying to sove this problem with my W124 for years, and just learned to live with it hoping that one day I will find the solution.
But most here claim that a vacuum leak will most likely be the problem. Check all vacuum hoses, replace those that are brittle, and look for any that have come loose from the fittings.
Also a bad MAS (Mass air sensor) could contribute to the problem. This is the cylindrical unit that resides between the air cleaner housing and the main intake.
2009 ML350 (84K) - Family vehicle
2001 CLK430 Cabriolet (71K) - Wife's car
2005 BMW 645CI (124K) - My daily driver
2012 Mustang V6 (60K) - Daughter's car