system is all new? any idea fuel trim is fine, tps and crank sensor data are fine, freeze frame data is where it should be, 45lbs of fuel pressure, new plugs, coils, coil cover. no current or pending dtc's- what are we missing?
I would suggest a Top Eng Clean and or Induction clean, The carbon can build up on the back of the valves and cause a lean condition. You may be able to set up a graph with your scan tool to show the arc of things like TPS along with O2 sensors. This can give you some idea of what is happening. Try checking the exhaust back pressure at that throttle angle I've seen stranger things. A colapsed map hose/ seal can do this too.