It could be the IP, but I don't know. It could be air, which hasn't been eliminated. Eliminating the IP would be simple: Just replace it (and you may still have a problem with air). Eliminating an air leak requires a process, but a lot less expensive than replacing the IP. If you have a leak, and replace the IP, you still have to fix the leak.
What I would do, knowing what I know about your truck at this time:
Plug in the block heater.
Connect a battery charger (charge, not trickle).
Install a clear line loop at the fuel return outlet at the pump.
Verify the
HPCA is getting power on a cold start with key ON. It's the other wire connected to the IP (not on the throttle, if it has a
TPS).
Come back after 2-4 hours after connecting charger and block heater.
Injector lines tight, and everything ready for a "normal" start. Crank engine in 15 second intervals (with 5 minutes rest). Watch the fuel return clear line loop. The fuel should push out any air, and you should not see more bubbles. If more bubbles, back to step 1: find/fix the leak.
It started and ran before, which means the IP and injectors are, at least, minimally functional. Something caused the fuel volume to diminish or stop enough to not continue running.