2005 4.8is - Random Rev @ Idle

[kskpsu]

Hello all,
Long time lurker, first time poster. I've got a random issue at idle, wonder if anyone else has experienced it. On a couple of occasions while my car is idling in park, the car will seemingly rev itself from just over 500 rpms to about 1,500. Then just as mysteriously, it will return back to normal and behave as expected.

I've searched the forums and find loads of info on rough idles in cold climates, and while I am in frigid Illinois, my idle before and after the rev is butter smooth.

Anyone have thoughts on root cause?

Car has 40k miles and is pampered (current on maintenance, only Shell V-Power, etc).