Ford focus losing power at 4k+ rpm?

[Spike]

Hi, i just bought a 00' ford focus zx3 standard from a family member, owned it for 3 weeks now. When i got the car i thought it was pretty quick being stock and all. After about 2 weeks the car seemed to loose power after about 4-5k rpm and sputter, it does it in all the gears but sometimes not till 6k rpm. This also started to happen after i ran a store bought fuel injector cleaner through the car, but my friend and i didn't think it had anything to do with it at first. So he suggested i replace some of the standard parts, spark plugs, plug wires, and air filter. They didn't fix the problem so we decided to look into the fuel system, so we replaced the fuel filter, the old one was real nasty so my friend and i thought it was possibly blocked and the cleaner cleaned the gunk in the tank. Well after we took it for a spin the problem appeared to be gone but the next day it started to happen again. We also hooked the car up to a diagnostic tool no errors or engine lights. I seem to think the fuel system is possibly dirty still after the new filter even, possibly the air intake, or exhaust system, or possibly the catalytic converter. Does anyone have any ideas of what the problem could possibly be?