I see a bunch of replies. Not sure why they are hidden.

 

Weird. A bunch just showed up. I'll see if I can trash the copies

 

Pulled out the whole TB and gave it a thorough wipe-down. There was a surprising amount of gunk still in there on the back side; definitely going to pull the whole thing off from here on out. Anyway, cleaned out all the **** in there and gave the IAC port some needed love. Pics attached.

Seems like it helped a little - engine is idling steady with a little higher RPMs and is LESS likely to drop far enough below 500 to be concerning when I shift out of gear, but it still happens. I did manage to get it to stall when it was cold: stopping at the top of the ramp from our parking garage, A/C off, and engaging the power steering in preparation for the next turn. But that was the only stall.

However, the weird RPM dance is still happening when I am MOVING in neutral. Stopped, at idle, the tach sits steady at ~650. Any ideas what would cause this behavior? Should I swap another of the sensors I replaced? MAP? Or could be fuel related?

 

Is your speedometer working correctly? The speed sensors sends a signal to the PCM that helps control decel, like an old dashpot.

 

Thats an interesting possibility and would seem to make sense with the moving-only symptoms... Speedometer seems to be working correctly, at least it's not jumping around or horrendously out of sync with my actual speed. I guess I could pull off the speed sensor and test it with a multimeter? I'm getting the feeling the root cause is electronic