I know, I know, I'm sure you guys have gotten stuff like this before, but I simply cannot find any other threads with this same issue and it's driving me absolutely nuts.

When the engine is cold (a minute or so after startup in the morning) and while coasting in D4 at around 20-30 mph on a slight downhill slope, the idle will bounce up and down between about 800-1000rpm rapidly for about 5 seconds and then stop. Doesn't happen when it's all warmed up. This isn't a typical idle issue involving the IACV or anything of that nature, as the IACV works fine and the car idles great in park and runs beautifully. All vacuum hose are plugged in correctly and are in good condition, upper/lower manifold gaskets and head gasket are brand new and properly tightened down, cooling system purged of air, new ECT sensor and sender, valve lash properly adjusted, timing all lined up. I could list other things that I can't think of off the top of my head, but you get the gist of it. The only existing/recurring code is the P0420.

Any pointers are appreciated! I REALLY want to get rid of this irritating problem!

 

Try warming up the car fully and see if it changes.
There is a grade logic in the PCM where the TC locks/unlocks when decelerating/off throttle. It could be the cold startup/open loop logic is just not as refined as the hot enigine/closed loop strategy.

 

So warm the car up to full operating temperature?

I read the live vehicle data on a scan tool while trying to figure out the P0420 code, and the short and long term fuel trims were both consistently below the factory spec of +1. Short term was around -4 or -5 and the long term was about the same. So I'm led to believe that the code is false, especially as the code never came back until I redid the IM gasket to seal off a bad vacuum leak (wrong gasket caused a P0505). So maybe the vacuum leak sort of "fixed" the overly rich mixture until I redid it, at which time the P0420 came back again. I've read from some sources that O2 sensors on their way out could cause the issue, especially considering that the engine is running so darn rich.

I forgot to mention that I tried putting the transmission in neutral while it was bouncing, but it didn't change anything.