If I can tack on to this... I've had a couple of issues like this, mainly the bouncing idle at cold start down to almost dying (though not quite) that goes away and runs normally after warm up. Finally got a CEL, which showed a 1421 code. Cleared the code and it came back once. Cleared it again and the car has been fine the last few days with only one little idle bounce at a stoplight, but not the extreme dip that it has done before.
I listened for the vacuum pump running at startup and it seems to be running okay, though I didn't check further than that.
I can see a small crack in my intake boot at that elbow where the smaller tube splits off. Could this be the source of the problem? In other words, is it possible or likely that the rubber boots are the only problem? I'll change them out this weekend if so. In fact, I need to anyway with the car at 170k driven in Texas heat. At that age and mileage, I'm sure the one crack I can see isn't the only one.