The computer is not properly opening the IAC because it is confused from your modifications. You can solve it by simple turning the throttle plate stop screw on the passenger side of the TB one full round. This will manually hold open the throttle plate and prevent the engine from stalling when you quickly let off the gas. Your idle speed will increase but you can dial that back down by using the set screw under the gray plug. I would set the idle around 900.
A slight twist on Shaun's idea is to just run the bleed screw out a little. As you do, the idle will start to return to idle slower, which should act as a buffer to keep the revs from dropping so fast.
Either way should work IMHO - we dont know your existing settings or how the car performs otherwise.