Hey scotty

I have a 2002 Mercedes ml320 petrol, v6 engine (12 spark plugs) with an issue. When i'm on a highway going over 80-90kmph, and a hill is coming and I put my foot down 50% or more on the accelerator, the car begins to choke like it doesn't want to go and the engine light comes on. But when I'm driving in a residential area say 40-60kmph and I put my foot down more it does pick up pretty fine like the issue isn't there. It mainly chokes when I'm on a highway going above 80-90kmph, and I want to overtake or a hill is coming. 

I noticed that when it chokes at this situations, the gearbox doesn't even gear down, so if im in 5th gear on the highway, and i put my foot down because a hill is coming, it doesn't gear down to 4th or 3rd for more power, it stays in its gear and chokes and the engine light turns on. 

I have replaced the crankshaft sensor, camshaft sensor, new coils and spark plugs, new fuel filter, afm is okay. Air filter is not blocked, both o2 sensors replaced. 

I know something like this is difficult but could you point me in the right direction? 

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