Goood afternoon and welcome to JustAnswer!. This is Ed.
If your problem happens within the first five minutes of engine start and then settles down, you probably have a weak heater on your upstream oxygen sensor. A slow-to-heat 02 sensor signal will resemble that of a very rich-running engine to the PCM (engine controller), prompting it into corrective action, which would be to take away up to 66% of its available injector pulse width. With the engine pared back that far, you will certainly have loss of power, stumble, sag and backfire through the intake at times. While the onboard diagnostics should pick up and code an oxygen sensor heater that's this weak, the cruel reality if it is that it doesn't happen often. As long as the electrical heater within the sensor still sort of works, the problem will evade detection.
To tell if your sensor is at fault, you could try this.
The next time it's in the throes of a loss of power, do a quick key cycle. RUN -OFF- RUN. Just pop the transmission into neutral while in motion (if conditions permit) and restart the engine.
What this does is to put the fuel system back into open-loop mode, which means the PCM will disregard the oxygen sensor signal for about 30 seconds and all the adaptive damage that had accumulated earlier will be erased. Everything goes back to zero, which is normal. If the engine immediately runs better (and it may now run OK for the rest of the day), it shows that the problem was being caused by an erroneous adaptive shift, caused by the upstream oxygen sensor, the one located ahead of the catalytic converter in the exhaust system. It'll be the one that sits right in the convergence of the exhaust streams from the two banks.