Not coming all the way down to set idle

I'll just throw a couple of things out there, that I'm sure you have already checked. These are for future readers using the search button.

1) the T-port sync: If the butterflies end up too far open, the Vacuum Advance port may be activated pulling in some timing, which will always increase the idle speed. The toe-tap may be just enough to drop the signal out.
2)the t-port sync: if the timing is too far advanced, then the throttle plates may park very closed to fully closed. In this position, the idle will be very sensitive to very small changes in throttle parking. It will usually be accompanied by a hesitation at throttle tip-in.
3) the T-port sync; If the butterflies are open a lot too far , the transfers may not want to quit. This usually manifests as the rpm taking an abnormally long time to return to the preset idle. But it usually does return to idle after some 2 or 3 seconds. Occasionally tho, it just hangs. The proof of this situation is to kill the ignition for a second and reconnect at just below the preset idle speed. If it then remains at the preset, that is the proof.
4) the weights in the dizzy can hang up when very light springs are used.Extra advance means extra idlerpm.
5) PCV. if the PCV does not park accurately, it can show up at idle.The PCV at this point is a controlled vacuum leak. If it doesn't park the same everytime, the idle speed will vary.This, however does not usually respond to a toe-tap.
6) linkages. Sometimes the links on the passenger side, make trouble. Especially the one to the secondary butterflies.There is usually a clearance spec for this link that is supposed to be reset every time the primary cracking screw is adjusted.