Stalling problems

I did not have any problems until one day when I shut it off it did not want to start back up. It seemed like it was not getting gas at first unless you hold the pedal to the floor then as soon as you let up it shuts off. The first code that popped up was speed sensor. I replaced that and both o2 sensors and nothing. I changed the cam sensor and the crankshaft sensor still nothing. I pulled the ECU, PCM and steering column out of another Jeep that was running just fine and still nothing. Before I changed that it said something about fuel injector eight and after I changed the computer the first code said fuel injector four and now it does not say anything about injectors, but keeps popping up both o2 sensors and speed sensor when they where replaced first. I am lost and need help.