my 20 is doing something similar. On come cold starts the idle fluctuates, like it's struggling, then once its warm it, fine. Only thing different is the E85. This is my 3rd tank but this last fill up was from a Shell station off the turnpike. Not sure what it is. Hoping it's just old gas and goes away after I fill up with 93 again. I hear others mentioning VCT solenoids but mine does not stall (yet) and it only doe sit at startup.

 

I am having this issue as well on my 2017 5.0. Finally threw a CEL but struggles almost like its starved for fuel as if you flutter the throttle it will almost die before it will idle up. But after 30 seconds of this - it runs like it was brand new and will only do this from an absolute cold start. Did it this morning at 30 F but then later in the day it started just like it should, smooth idle and it was sitting outside arena at 20F. No idea.

Code I got is P050B - Service manual says basically nothing - no vacuum leaks that I can detect, cleaned throttle body, new air filter, ran some water remover and 2 tanks of 91 through it.

Asked a local garage that has some decent talent working there and they were a bit confused. Wondering if it is the start of a VCT solenoid failure but I change oil regularly at 100 engine hours no matter what as the truck does a lot of "idling" around when we are hunting the dogs looking for tracks.