Okay, I have a 2004 Ranger XLT 2.3, with a manual transmission; 55k miles.
This is a problem that seems to come and go as it pleases. Here it is:
Yesterday, I drove to the store for a sack of groceries. I parked the Ranger, and went inside. Fifteen minutes later, I got in the truck and hit the starter; it cranked like mad, but wouldn't fire. So, I tried it again... several times... and once in a while it seemed to want to sputter but wouldn't run. Gas tank is half full, spark plugs have around 15k miles on them. After half an hour of attempts with waiting time between attempts, I locked the truck and called for a ride home. Meijer's store manager told me it was okay to park it over night.

Today, I got a ride back over to the store, inserted the key and it started as if nothing was ever wrong. Whiskey Tango Foxtrot!
This same situation has happened in my driveway a couple other times, several years ago; since it never happened again, I simply forgot about it. Well, yesterday, it happened again, and I was stranded (with a bag of frozen food). The next time, it could be God knows where.

I did a lot of snooping and studying on my Ford Shop Manual and Wiring Manual, and I think it might be a flaky ignition switch... the one mounted under the dash, on the base of the steering column. The dealer doesn't stock the switch, so I ordered a new Motorcraft SW5011 switch from Rock Auto; when I get the old one taken out, I'll see if it's intermittent... but I wonder if there are any other potential problems that might be lurking?

Does anyone else have any other ideas to toss out? Thanks!

 

So it only happens when engine is warm or partially warm?

Never ever when cold?

That wouldn't be ignition switch, it is a mechanical device and warm or cold engine wouldn't matter.
If the lights on the dash come on then ignition switch would be a long shot
If CEL(check engine light) comes on then computer has started up.

Each time you turn on the key you should hear the fuel pump Hummmm, for 2 seconds
Do you remember hearing that?

Check the Crank position sensor(CKP) on the main pulley, about 7:00 position, see if it is loose, long shot.
CKP times spark and fuel injection for the computer, computer doesn't even know you are trying to start the engine until it sees CKP Pulses timing when #1 is at Top Dead Center.
This sensor doesn't tend to be intermittent, it can fail out right, or be damaged, but intermittent is not a common failure

There really aren't alot of things that can prevent an engine from starting if starter motor is working.
No fuel
No spark
No compression

No compression is a mechanical failure, so wouldn't allow a restart at anytime, so off the table

No spark is possible but it wouldn't usually "act like it was trying to start" at times.
I believe the Duratec 2.3l uses COP(coil on plug) spark so all 4 coils would have to fail for a no start or computer would have to fail, and I doubt it would restart ever, so long shot.

No or low fuel pressure can act as you describe, "tries to start" and a fuel pump is an electric motor that can be effected by heat, it does get warm, the gas in the tank cools it down.
Do you remember if the problem is more likely to occur when tank is at or lower than 1/4 full?

Because it is an intermittent problem there are no real tests you can do except at the time the problem occurs.
I would get a spray can of Ether(quick start) to carry in the truck.
When the problem happens, open the hood, open the air filter box and remove air filter, spray some ether into the intake
You can leave air filter out and box open.
Try to start the engine
If it fires up and then dies Spark is working but there is no Fuel
If it doesn't fire up then spark is the problem.

50/50 test


It is not good to spray anything into the MAF(mass air flow) sensor, but once or twice for testing is OK, just don't make a habit of it.

Another long shot but known to occur on the 2.3l is a bad TPS(throttle position sensor), but this is easy to test for when the problem occurs.
With a no start, open the hood and locate the TPS(good to find it before hand), unplug its wire connector.
Try to start
If it starts replace TPS, if it doesn't then try the Ether in the intake, you can plug the TPS back in.
All fuel injected engines have a "Clear Flooded Engine" routine, if you press gas pedal to the floor and hold it down before engine is started the computer will see 5volts from the TPS(foot to the floor) this tells computer to start Clear Flooded engine routine, computer turns OFF fuel injectors but leaves spark on, so an on purpose no start allowing you to "dry out" a flooded engine.
If TPS has an internal short then computer may get that 5volts and shut off fuel injectors.
This wouldn't tend to be intermittent, so a very long shot but not a no shot