intermitent low power (no turbo), limp home mode, no light

The day after I purchsed this vehicle the check engine light came on and it went into the low power mode. Turning off the engine for a few minutes will clear the low power mode but would not turn out the light. Eventually after many weeks of taking it back to the dealer and different sensors and various remedes it was taken to the dealer where it was determined the waste gate controller was bad. Unfortunately the waste gate controller and the turbo itself are one unit. The whole turbo assembly had to be replaced at a cost of over $5000. Fortunately I had purchased an extended warranty on the vehicle. This seemed to fix the problem for about a week. Now the low power mode happens once or twice a week but no check engine light. Otherwise the car runs great. You can still stop and turn it off for a few minutes and it will reset. Sometimes this will last several days or maybe several minutes. It appears to be very random. No one knows what to do next. This seems to be a common problem with these 3.0 turbo diesels. I've seen many references to the same problems posted on the internet whether it be in a Mercedes car, suv or a Sprinter Van.