Then go get the codes.
Resistor connectors can not be diagnoses b/c theylook fine..they are an electrical resistor and a meter/scope does the looking...not the human eye..

Which us why all the TECHS that are BenzHeads know to change the connectors when they change the plugs, wether they look fine or not...that was my point.
Do a Search on plug connectors on Waste Spark [ DIS} systems and you will see why.
However , with you rpm complaint. I would get the codes first, for sure.
If he did that, post waht codes he retrieved.

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A Dalton