Try a differant dealership/mechanic. I have been in the service business for better than 30 years (computers) Some techs are simply not that good at diagnosing and troubleshooting. If i were to guess as to the fault,the turbo is not varying output correctly.
A worthwhile article.
http://www.forddoctorsdts.com/articl...icle-05-02.php called
Critical Technician Shortage?