F200 puzzle

[Alaskalodge]

I have a 2006 F200. When the motor is completely cool,(start up and running for the 1st 15 minutes or so), it runs beautifully. After about the 1st 15 minutes, it slowly starts to drop rpms. For example, I will be running at 4800 and it will drop to 4400. If I push the power trim button, located on the controls, for that motor, it will temporarily restore the rpms and you can even hear and feel the engine pick up speed. This will go on until finally the motor starts to cough and pop.
If I take the cowling off at this point, within a matter of minutes, with more air flowing over the motor, the problem will disappear entirely and the motor will run fine.
Can anyone give me a direction on what part may be failing to cause this phenomenom? Sensor? Wiring? Fuel?

Thanks.

[robert graham]

Just maybe there's some type of obstruction in the air inlet in the upper cowling?...like bug nests or bird nests?....check it out...

[Alaskalodge]

No obstructions. I have open everything up down to the fuel pumps to run tests so definetly no obstructions

[rodbolt17]

when you tag the trim, system voltage drops.
the ECU compensates by adding to the injector ON time.
I would carefully test fuel rail pressure at the failure RPM and load.

other than that, a failing ISC valve.

intake backfire s normally caused by a lean A/F mix in the intake.
either due to loss of rail pressure or a clogged injector screen.