A would say no on the Flex fuel sensor, never heard of it restricting fuel flow, and restricted fuel flow usually shows up at higher speeds, i.e. vehicle can't maintain 60mph, it loses power as fuel flow tampers off and pressure starts to drop, you would also get Lean Codes for both banks.
Fuel rail on '99 Ranger should have minimum 55psi for months and months if it sits, so you have a leak in the system, it is losing pressure when pump is off.
On the rear of the fuel rail is the Pulse Damper, it has a vacuum hose attached, remove this hose and check it for fuel, if there is fuel then replace Damper.
Fuel would be leaking out and into the intake via vacuum hose causing a "Flooded" engine condition so a hard start.
A leaking fuel injector could do the same thing, pull out all the spark plugs and have a look at each one, if one is blacker than the others then I would suspect that injector is leaking.
There is also the Check Valve on the Fuel Pump
Have a read here: fuel system check valve
Sageport thinks debris was holding open his Fuel Pumps check valve so he was losing pressure, while he did have to pull out the pump he did fix it without having to replace the pump