it sounds like an iac problem. i know EVERYONE says its ALWAYS the iac, but i have good reasoning!

the iac controls how much air is still allowed to enter your engine after you are completely off the throttle. when you rev high with the car in neutral and then let off the throttle, the iac closes almost all the way or else fully to allow the rpms to drop. then when the rpms get low enough the iac opens again almost fully so that the air "catches" the rpms and holds it steady.

on my car, ill rev it up, and the rpms fall quickly down to 1000, then slowly drop down to 850-900, and stay there.