I'd say either IAC or fuel injector issue. See, your opening the throttle, you have air, yet you have a slight drop in the revs before it goes up.... So what I'd say is either the IAC is screwing up its readings for a second or your injectors arent squirting when they should. Replace the IAC first and let us know how it goes. I'm assuming this may happen when you rev up and let go of the gas too? Will it drop below the idle then go back up?