The Ford Computer runs the fuel pump.
For safety reasons the fuel pump will only run for 2 seconds when key is turned on, it won't come on again until engine RPMs are above 400(engine is started).
The fuel system in a '99 should have 60-70psi of fuel pressure when engine is running, and stay above 50psi when engine is off, it will hold that pressure for a few months, not minutes, hours or days.....months
If you are losing fuel pressure when truck sits for a few hours, then the 2 seconds of run time for the fuel pump won't provide enough pressure to keep engine running.
To test for this loss of pressure try turning on the key, then count to 3 and turn it off.
repeat 3 times, key on, key off
Each time key is turned on you should get 10-15psi added to system, don't worry you can't "over pressurize" the system.
Now try to start engine.
If it starts up normally then you are probably losing pressure when truck sits, if you don't smell gasoline then the check valve in the fuel pump is most likely the problem, it is part of the fuel pump not a separate piece.
If engine doesn't start then problem could be IAC valve, it can be cleaned, or ECT sensor.
IAC(idle air control) Valve opens up all the way so engine can get enough air to start, then closes to set Cold Engine idle, high RPM, until engine warms up.
Computer runs the IAC Valve, it is a moving part so can get dirty and stick closed, causing starting and running issues.
ECT(engine coolant temp) sensor is used only by the computer, it tells the computer if engine is cold or warm, a cold engine needs more fuel and more air to run, that's what the Choke Plate did on a Carburetor engine.
Fuel injected engine can't use a Choke Plate, no Jets, so the computer must run a Choke Routine, more fuel more air, until engine warms up.
If ECT sensor is "telling" computer that the engine is already warm when it is really cold then engine would struggle to start and stay running until it was warmed up.