[Blue Bowtie]

Fuel pressure needs to be 58 PSIG minimum to start. That's the pressure needed to overcome the poppet valves in the injector/spider assembly. Since the pressure seems to drop off after it's been running briefly, I'd suspect a resistive electrical connection is dropping voltage in the fuel pump circuit. that can be anywhere from the bussing in the underhood electrical center (UEC) to the pump itself. The easy thing to do is swap the FP relay with the horn relay to eliminate that possibility. After that, there is an electrical connector just forward of the fuel tank. That can be disconnected and inspected, and if necessary the connector can be back-probed while connected to monitor fuel pump voltage. If the voltage drops off at that connector, start looking forward to the wiring, relay, and UEC connections. If the pump voltage at that point remains constant, look at the connector directly on top of the tank. It has a bad reputation, and there is a factory TSB for repairing it due to overheating and resistance.