how to fix a 220CDi?

Sir.

I am a senior lecturer in motor vehicle at a local college and we have an ongoing problem with the above car and wonder if you can be of help to us. The runs very well for about 4-5 miles then suffers a power loss…as if the turbo is not operating! If you stop the car and turn off the ignition then restart the car it will run again ok but only for a mile or so. We have purchased a turbo intercooler hose from mercedes recently but still no good>
We have also cleaned and disconnected the EGR valve and also changed some small hoses from vacuum valves. We have put a vacuum gauge onto the hose to turbo actuator and that appears to work ok, we have also changed the air cleaner element, the fuel filter, and even disconnected the Mass air sensor but all attempts fail and the car will be ok for a few miles then lose power. It is drivable but can be dangerous if overtaking !
Our code reader does not show any faults and there are no dashboard warning lights coming on. We have checked every hose or connector that we see but frankly we are baffled. Do you know of any faults that are common to this model/engine as lots of hair is being pulled out by 6 -8 lecturers at the moment…..they have all had an input but cannot find the reason for the fault, perhaps you can give us some advice.

Look forward to hearing from you

Regards
minimod