My 2000 Sable idles up high when starting, then the RPM's drop to like 500!

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I've replaced numerous parts on this car. Plugs, wires, the coil pack on #3, IAC, MAC...but when I start it in the morning, it idles up high then drops down to about 500 RPM's and will choke out if I don't step on the gas. There is a clicking sound from time to time near the throttle body before the idle catches and revs back up. It's also sucking gas like crazy!!! The check engine light is on and says the IAC is bad even after putting on a new one...is there a leak in the vacuum? HELP!!!