If you unplug your iac and drive it around, does it still happen? IF not, its dashpot for sure. I know you said it doesnt seem like dashpot but it sounds to me like dashpot preposition. Basically when you come off of closed throttle, the iac is opened to a specific amount and that amount is too much. I would try reducing the 2 lowest preposition points by 10% and see if it helps. If you have already tried that, im not sure what it would be.