I have a 2009 M320 diesel with 56k miles. I am having intermittent problem when driving at a constant speed around 1800 rpm, usually going up a slight grade. Feels like engine lugs down and cannot shift to lower gear, or is being fuel starved. Whwen you rev up and push rpm up to 2500 or 3000 it smooths out. MBZ dealer changed fuel filter about 5k ago and replaced NOX sensor/valve 2 weeks ago. Engine light does not come on. Anyone have any ideas??