Hi Susan. I'm niot an expert but have had similar problems with my VW Passat 2.0 sports. Is your Merc a turbo? From what you say, you may have turbo issues, injector issues and/or boost reguation issues. Mine went into limp or safe mode to protect engine. You lose power suddenly and it can be dangerous. On mine, the glow plug lights up and I get a warning message. Get a diagnostics done for fault codes. A clean/flush may sort it out but maybe only for a while (sticky vanes due to oil, etc). It may be you need a new turbo. Hope this helps.

Hi Susan, not sure if you have resolved the problem but let me tell you it is not a turbo issue or anything previously mentioned.

I have an ML420 on 57 plate and had exactly the same problem as you. In particular when I used the car for over say 30 min journeys. What would happen would be a loss of power when I pushed the acelerator.

I had taken the car back to Mercedes many times before they told me that it was a system fault. Apparently what happens is the car thinks there is an engine critical problem and shuts itself down, moving into limp-home mode.

This is an ML420 fault and dont let Mercedes tell you otherwise. I am sure we are not the only ones suffering with it.

Anyway, eventually Merc fixed it (they told me the cost would be £690 but I would only need to pay £90 to cover the labour cost. Since it has been done (6 months?), I have only experienced the problem a couple of times.

Take it back to Merc and get them to do something. If they cannot identify the problem let me know and I will give you the contact details for my local dealership