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Sometimes parts we have replaced and think are good are actually not (I guess that's why they have warranties ;-).
If it feels like is running out of gas, then it probably is. Check fuel pressure at the rail. If not about 40psi, check that all pumps are getting battery voltage with the engine running. If voltage is good, but pressure still not around 40psi, block the return line from the fuel press regulator and retest pressure. If not over 45psi, then the pumps are not doing their job.
If fuel delivery is OK, then you'll need to run a cylinder balance test to see which cylinder(s) is/are not performing as well as the others, then go from there and pinpoint the cause - plugs, wires, mechanical.