This has happened to both of my vehicles.
1st trucks problem: fuel pump was on its way out, so I replaced it. 2 weeks later it was gutless and ran like crap. swapped out the fuel filter and it ran like a top. turns out my tank was rusted on this inside(thats what happens when a truck sits in the backyard for 2 years). I had to replace the $5 fuel filter every month or so to keep it running smooth.
2nd trucks problem. O2 sensor. . . . I drive a Pathfinder so a new one would have cost me nearly $200 if I had to pay full price. once I swapped a new one in there I got about 100km more out of a tank of gas t'boot.
there are so many reasons this can be happening and I don't feel like reading 4 pages to find out if you know what the problem is or not yet.
I do have one thing to say though, how the fuck do you claim to know how to replace all of these parts and say you know what a distributer cap is but you don't know what a rotor is? and I'm not talking about a brake rotor.