See if you can do what Mr.Kelly suggests above. If you can sweep the pressure with the adjustable FPR then I would say the FPR is working as it should be. Check the vacuum line on the FPR for fuel as that is an indicator that the diaphragm is damaged and you can usually get rebuild kits for them. From there I would change the filter as that is cheap and easy. If the filter is still the stock style it will be a Motorcraft P/N FG-1060. It is located above the passenger side of the rear end in front of the fuel tank. The fittings on either end have some aweful clips that hold the lines on the quick connections of the filter housing.
LMR has a pretty good video on replacing them:
View: https://www.youtube.com/watch?v=7VkHaqnqy2Q
If this does not fix the issue then my next guess would be the pump so that means dropping the tank.