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I would first verify that there's no vacuum leaks going to the FRPS. Then I would check the fuel pressure with a mechanical gauge versus the electronic sensor. Remember that it's targeting 39psid; that's a delta, versus the intake manifold pressure. So when it's at idle pulling vacuum you're only going to see something like 30psig (versus atmosphere), and at WOT, it should be just about 39psig, on the mechanical gauge. The sensor is manifold referenced so it should stay at a relatively steady 39psi, but you'll have to math out the mechanical one.
If you can datalog or pull freeze frame data with an OBD reader, that might help you determine what the sensor is reading as well.
With that information you can determine if there's a fuel pressure issue, or if you just have a faulty sensor (or wiring).
Note: When the engine heat soaks (usually IAT >100 degF or higher), the ECM will command a higher rail pressure, closer to 50psid.
psid: psi delta between two values i.e. fuel rail pressure vs. intake manifold pressure
psig: psi gauge, pressure referenced to atmosphere
psia: psi absolute, pressure referenced to absolute vacuum