Prevalent cause of idle surging with this particular engine is a vacuum leak on one or more of the hoses.

You can possibly trace it yourself, but a mechanic who is familiar with the vacuum hose routing on the W124 can do this rather quickly.

I endured the surging for about 4 years before I had my mechanic trace the cause and fix it in about 15 minutes.

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2009 ML350 (106K) - Family vehicle
2001 CLK430 Cabriolet (80K) - Wife's car
2005 BMW 645CI (138K) - My daily driver
2016 Mustang (32K) - Daughter's car