Only real suggestion I could give to your techs is to check for vacuum leaks...idle surging and rough idle is pretty much a symptom of that. Check the intake manifold and vacuum lines for leaks. You can spray a bit of water near those areas to see if the idle changes...if it does, you've found the leak.

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2009 ML350 (106K) - Family vehicle
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2016 Mustang (32K) - Daughter's car