It’s difficult to say what could be causing the problem, because it would seem to be intermittent ad they are the hardest problems of all to solve. By turning it off you are in effect rebooting the computer, so it will run normally until the next time it detects the problem. To have any chance of diagnosing the problem it needs to be evident when the mechanic does his check. If it’s not there’s nothing to see. It could be a number of things; all you can do it to persist in the hope that it will show up during a diagnostic check.

I've driven both and much prefer the Sprinter. The "manumatic" in the Fiat is not good and I prefer the way the Benz sits on the road, how it performs over bumps and its resistance to crosswinds.

There are a number of possible explanations, from the turbocharger, to the fuel pump, and the injectors. Your dealer should be able to diagnose the problem, if not, take it to another dealer. If that also fails take it to a diesel engine specialist.