The rear throttle plates should be closed at idle. Often the rear plates will get carboned up and stick slightly open this causes high unstable idles.
From the top hold the secondary air valve open (WITH ENGINE OFF) and use a hard wood dowel (or reasonable facimile) to push the plates fully closed and try it again. If you want to see if the close properly use the same dowel to push the plates slightly open. If you do this with the engine running it will probably die. After you release the plate it should return by itself without help and idle like it did after you manually closed the plate above.
Owner 24 bay BSC
Bosch Master, ASE master L1
26 years MB technician