I have a 2009 1.6 TDCI and am having similar problems, mainly when on the motorway and cruising for sustained periods. I have an OBD reader and my car shows no faults, but the turbo boost does drop when the fault occurs. Despite no faults indicating if I clear the fault codes and stop the car the fault dissapears for an undertermined time. If I give my car a good run then the fault doesn't replicate. I have cleaned the MAF and all the indications of sensors are correct for the engine with the exception of the turbo boost when the fault occurs. Does anyone know if the speed sensors feed into the engine management. I believe that the ABS sensors double as speed sensors in the Fiesta, could this be the issue?