i have a 1998 ford galaxy 19tdi 1z engine ,when driving and you try to speed up it lost power ,its stays around 50 mph sometimes you turn engine off and on speeds comeback a few seconds then you lost it again ,i have had it on 3 test machines and come backs with no faults ,the air flow meter ,tubro and the egr have all been check and all working ,any help would be very greatful