Here's an odd one for you. Friend of mine has a ML320 with the V6 engine and its down on two cylinders...codes say faulty coil packs. His local Bosch garage have replaced the offending coil packs and have replaced all 12 spark plugs, cleared the codes and the car fires up and all seems well, until it resorts to running on 4 cylinders...both rear cylinders not firing...codes say failed coil packs again.....coil packs have then been moved around and the same sequence starts all over again, ie runs fine, then resorts to running misfiring on the rear cylinders. Any clues to whats going on???