I had this same problem the last time I had my intake off. It would idle at about 3500 RPM's with the throttle plates fully closed. It turned out to be the front W shaped pipe in the engine compartment, it was not totally sealed to the intake. Even though it had gaskets they were old and cracked. I replaced them and it started right up and idled at 800 RPMS. I doubt your butterflies are sticking. Just pop off the front air horn (W shaped pipe) and watch them. good luck David