1990 f150 sputters after warmed up.

You need an auto electrician with a pro grade scan tool computer to record what happens after 15 minutes to make this happen. The $5k machines can do that.

Does the temperature gage on the dashboard show that the car is getting warm?

there is problem with relay PGMFI is under left leg.

I'v got one with over 301,000 miles on it. IT sounds like you need to replace the crank sensor. The balancer will have to be removed for that. Next time it stops running spray or pour some water on the crank sensor and see if it will crank. Did that to mine and it started right up until it got hot again.

Injectors shouldn't effect each other
You may have a bad coolant temp sensor, and it doesn't go into closed loop.
That is when it is warm and the O2 Sensors control the injector timing.
A bad thermostat also
Egr Valve ? Rusted out exhaust tube to egr
Vacuum Leaks?
Exhaust Leaks,near sensors?

I would suspect fautly spark plugs.

try replacing your EGR Valve, or your Oxygen Sensor

do a compression test

Sounds like a spark plug wire or plug problem, how many miles on truck?

Have you ruled out vacuum leaks?