After my truck sits overnight it starts immediately and goes to fast idle for a few seconds, then ithe idle starts dropping until it gets to approx. 400 RPM.
If I drive off immediately it will correct itself in a short while, otherwise I need to baby the engine speed until it runs on its own - usually five minutes.
I cleaned the MAF and performed a MAF tap test with no success.
I would clean the idle air control valve and check both the engine coolant temperature sensor and air temp sensor.
These engines are known for leaking intake manifold gaskets
You have the classic symptoms
Runs rough after starting up cold, then smoothes out as the engine warms up
I’ve seen these symptoms, on your truck, because we have tons of them in our fleet, and I’ve replaced a few intake gaskets, with great success
I agree that you have all the indications of a leaky intake gasket set. Only takes a few minutes of warming up to get them to seal up and the engine starts to run properly.