Finally starting my 5.3 swap!!

[BR3W CITY]

It was running fine before right? I watched your video from the previous page, but the audio from in the cab isn't much to go by.

The perennial question; scan for codes?

IAC is controlled by a stepper motor, not a vacuum source or mechanical linkage like older vehicles. If you have a problem with that, for the most part its in the tune, unless you have another handicap with it. Make sure the maf is plugged in and sitting correctly, make sure you don't have an obvious vacuum leak. A very dirty tb can cause issues with the air gap on the blade, or worse if the bleed hole is plugged.

Anything else change? This doesn't seem like an issue from first start right, since you've put a couple miles on her.