2003 / 2004 ML350 engine problems

[johntastic]

Hi i have a 2003 ML350 with 195000 mile and has ran like a champ,
6 months ago i had a misfire issues so I changed plugs and it ran fine, then the rough idle /misfire came back I cleared codes and it ran fine for weeks then misfire came back, cleared codes again and ran fine for weeks then came back,etc,etc,etc, finally took to Mercedes / then to another Mercedes mechanic
here's what we did.... new plug wires new plugs new ECM computer, new cytolytic converters still codes of misfire, was told needed new engine s buy used 3.7 from a 2004 ml350 with 60,000 miles put all the new stuff on it
engine run perfect while idling for hours and hours but when drivin after about 2 miles misfire comes back , clear codes and drive 2 miles misfire comes back!!
the thing starts perfect and will idle all day rev it up its stong as hell but drive two miles and misfire again, I believe my old engine is fine since it did the same thing, DOES ANYONE KNOW ABOUT OR HAD THIS PROBLEM! PLEASE HELP!!
SOS!!! MAYDAY MAYDAY!! I'M AT THE END OF MY ROPE !!! WHAT THE HELL IS GOING ON WITH IT AND NEVER MERCEDES OR MY FOREIGN MECHANIC CAN FIX IT..

[Jorn]

Throttle body?

[ILUVMILS]

You don't say which cylinders are mis-firing. FWIW, I've seen faulty O2 sensors knock out an entire cylinder bank........