14 Ignition pickup was erratic - Ignition Systems
18 (R) Check base timing & advance function - Timing Tests
14 means the tach signal, PIP, that the TFI module is sending to the computer is not steady, computer uses this signal to time injectors
18 means 14 and base spark timing may not be correct
But the running and then stalling reads more like a fuel issue than spark issue
Do 50/50 test
After there is a no start spray gasoline or starting fluid into the intake and see if it starts
If it starts and dies then Fuel is the issue, spark is still working
If it still doesn't fire up then yes its 100% spark issue
You could have a clogged fuel filter or pump filter in the tank
So truck has to sit a bit before enough fuel can filter in, then it starts and runs until that fuel is used up, and each time it starts it runs for less and less time, until its all used up until it sits again
If 50/50 test shows 100% spark issue
Then check the wiring on TFI module for frayed ends
Also near distributor follow the TFI wires and you will find the SPOUT connector, looks like this:
https://www.fordforumsonline.com/attachments/spout-jpg.866/
The plastic end pulls out, this disconnects Spark Advance, so you can time this engine with a timing light
Makes sure its there and that the plug is there, pull it out and put it back in to clean connections