how can i fix my vw sharan 1.9 tdi?

car lacks power, it will be driving fine one min then just shuts down as if turbo not working, turn engine off and start up again will drive again for a few more miles without doin it, have changed airflow meter and air temp switch and its not that, have had diagnostic plugged in and it comes up wiv map sensor fault but cant find that sensor and no one seems to no where it is, can anyone help?