So I have a 99 ranger 4.0 xlt automatic and it was a rollover truck that I had swapped the cab on and put a wooden flatbed. The cab was a junkyard cab in good shape and the truck ran fine. It started to slowly develop hard starts where it stumbles really hard and you have to barely play with the gas to get it to idle past 600 rpm but when it "comes to life" it's idles fine runs perfectly and has no issues. When I got the truck on the road it started to bog under load always consistently about 2 minutes into my drive then I got to repeat the trying to get it to stop stumbling but after that I pull back onto the road and it runs perfectly the rest of the way. Didn't touch any of the factory harness, double triple, quadruple checked for loose or missed connectors and it has a new IAC, MAF, TPS, fuel pump, fuel filter, air filter, and probably a few more parts I'm not able to think of right now these are not remanned or cleaned but NEW. All vacuum lines are hooked up and for sure I sprayed and sprayed starting fluid all around the lines and intake manifold makes for sure there's no vacuum leaks and I have a good 65psi to the fuel rail with no bleeding off. Help would be well appreciated I am usually very good with diagnosis but this one has me stumped. Have replaced most of what I could guess would cause these symptoms but have been wrong so far and it's getting expensive so any ideas or input or directions to look would be highly appreciated, thank you.
EDIT: so I found my issue, the truck ran fine before the cab swap. What I didn't take into account was the new cab came off a 3.0 and the frame I put it on has a 4.0. So the PCM wasn't switched and I ran it like that so the computer was looking for a whole different motor. This caused random rich/lean conditions but no serious codes and it ran terrible. This caused symptoms similar to all the sensors I replaced and mimicked them but none ever fixed the problem. So for future reference if anyone swaps cab and has these issues swap the pcm it's the problem. You will have to have 2 same year keys and the program to erase and reprogram the keys or the anti theft will not recognize the pcm.

 

The PCM is in the firewall. The original cab was still around so you could get the original PCM out? THAT'S LUCKY

 

Yeah got really lucky I did it myself and kept old stuff around knew I'd need it lol