Apologies if this has already been mentioned , but have you , while experiencing these symptoms , had a laptop onboard connected and running Protune to see if anything relevant is displayed ?
Just for fun I have had a passenger hold a laptop in view as I drive, to better understand the mapping screen and how it changes while driving.
In your case I am wondering if something transitory might be happening that is not recorded as an error code, but might be visible when the high idling happens ? Long shot .... So like your own rolling road test.
