High erratic idle issues, Idling between 1400-2000 RPM Options

[blu94gt]

So here's the deal with the idle, I searched and couldn't find too much helpful info:
Been having some high idle problems for a couple months. Car will idle very high, not go down when the engine warms up, etc. Put up with it for a few weeks since I don't do much stop-and-go driving. Get stuck in traffic one day in the beginning of January and come to figure out that I can rev the engine up with the brake pedal, close to 2500 RPM just from pumping the brakes. So I figure I have a leaky brake booster letting more air into the engine, so I pull the line to it and cap the inlet on the intake manifold. Idle immediately drops from a pretty consistent 1800 or so to 1100-1200. Fine, that's better at least.

So after that I cleaned my TB again, sprayed some carb cleaner in the idle control, and no change. Car will idle high, and every now and then after I turn it off and on again it will idle around 800-900 RPM. Lately it's been idling between 1400-1800 most of the time, and sometimes will dip down. My gas mileage has been around 23-24 mpg, can't find a vacuum leak, and the car is running really strong otherwise. New plugs, wires, distributor cap and rotor just last week. Oh and the car is a 99 GT, with a stock 5sfe, almost 180k miles.

Just want to get some input before I order an IAC unit for a few hundred bucks.

Cliffs:
idles between 1400-1800 even when completely warm, driving for over 2 hours
brake booster disconnected, was causing idle to be even higher
every now and then will idle closer to normal around 800-900
can't find a vacuum leak
replaced plugs, wires, distributor cap and rotor last week
think it's IAC?

[azian_advanced]

everything you say seems like it's an iac valve issue except how it randomly idles back down to 800-900 every now and then.
i'd start by taking off the iac valve anyway and clean it if its clogged with gunk or if the valve has trouble turning. i just did mine not too long ago and it wasn't too difficult. it solved my low idle issue on cold start up because the valve was stuck shut so i'm guessing yours might be stuck open??