Hello everyone! I'm new to this forum.
Just purchased a 2008 RR HSE with 46,000miles. Something I didn't notice during my test drive ...
When coming to a stop the revs drop to the 100-200 zone then pops back up to idle (around 700). When I try to gently come to a stop, the revs drop to about 400-500, then back up to idle. When I shift to neutral when coming to a stop, this does not happen. What could be going on? Thanks in advance for the help!

 

You have a dirty throttle body. The Electronic Throttle is dirty and is having a hard time maintaining proper idle as a result.

This is easy to clean, simply remove the air tube going to the throttle, prop open the throttle plate and with a solvent soaked rag, wipe all the black gooey stuff out from around the throttle. Don't get tempted to spray into the throttle, let the solvent in the rag do the work. Spraying into the throttle can allow solvent to wick its way through the throttle shaft bushings and damage the electronics.
Put it all back together when clean and the idle will correct itself and a few minutes if not instantly.

BTW, probably a Youtube video on this.