Well, after quite a bit more digging, I am pretty sure it's not the IAC valve, or even the fuel/air mix at all.

I hooked up the Torque app and noticed that my timing was going way off when I'm getting the stumbling. When running right, it was about 5-10 degrees positive (I presume before TDC), but when it started stumbling, the timing was 15-20 degrees negative (I presume after TDC).

Given the weird timing, I decided to research the crankshaft position sensor, since I read somewhere that it's part of the idle air control system calculations... turns out my crank position sensor isn't measuring within the specs. It's got zero resistance (open circuit), unless I run a magnet by it, then it gives some resistance. It's supposed to be 1.6K-2.7K ohms cold, or 2.1K-3.2K ohms hot.

Strangely enough, I don't have any code for the crank sensor, just the P0505 idle air control system malfunction. I don't seem to be having any issues with the RPMs being reported by the ECU either.

I have a Denso crank sensor on order and should be here in a day or two... I'll update with my results. LOL