lose of fuel pressure after 20 minutes on engine load

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1999 escort lose fuel pressure after 20 minutes when under load, engine vaccum is 19 in fuel pressure key on 50 psi idle 34 psi and pump driving 44 psi. car run great when cold. I have installed a new filter. Have used parts installed ecm, pcm, throttle body throttle position sensor fuel regulator idle air control valve mass air flow sensor intake air sensor water temp sensor. I have no no check engine lite no trouble code in the ecm