Idle speed control

Hi Folks, I need some advice.
I have a 92 buick roadmaster estate wagon with 204k miles on it. For the past couple of years I have had a continuing issue with the idle
speed. Normal idle speed runs about 600 rpm in drive.
The idle speed will be about 1000 rpm in drive and about 1200 rpm in neutral after I start the car. The high idle speed remains this way until I stop the car, place the transmission in neutral, and let the idle speed go to 1200 rpm. It will take about 30 seconds for the idle speed to slowly return to a normal of 600 rpm. The idle speed will remain correct no matter how long I drive (hours). The high idle will return if I turn the ignition off, and restart the wagon.
The high idle speed is a bit of a pain around town since you must use the brakes lot more. A high idle of 1000 rpm will keep me going about 20 mph around town.
I have change the plugs, coil (for another problem), replaced fuel injectors, and most importantly, changed the idle speed air control (twice now). I have also cleaned the spindle seat with carb cleaner and a brush. The high idle speed eventually returns after a couple of months. The OBD1 shows no error codes.
I am thinking of changing the Idle speed air sensor again but it runs about $35.
Does anyone have some suggestions to help me keep my idle at the correct level?
Thanks for any ideas, Dan
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Is the replacement of the idle air control valves been to combat this problem? I am thinking of a few possibilities. The throttle position sensor could be reading high, telling the computer that the throttle is opened, the throttle shaft and/or plates are binding, you have a vacuum leak, or there is possibly and idle learn procedure that needs to be completed. You said there are no codes, was this done with a code reader? If you are able to see live data, check the tps voltage when the problem is occurring, it typically runs around .4 to .5 volts at idle. Check also to see that the throttle lever is against the stop, and that the tps is not loose and floating around. It is located opposite the throttle lever on the body. Also, if you can read live data, check the engine coolant temp at cold start(it should be within few degrees of ambient), and monitor as it warms up. Ultimately should reach approx. 200 deg., if not you may have a t-stat that is not allowing full temp to be reached. Hope something out of this is to your benefit. Gerald
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On Wed, 16 Jan 2008 21:06:27 -0800 (PST), Certainly

problem. The replacement would work for a couple of months but the problem would then become intermittent and slowly the high idle speed would become consistant.
I am thinking of a few possibilities. The throttle position

not idle at all. I also had an error code.
the throttle shaft and/or plates are binding, I will check this when the weather becomes much warmer. Forecast for this weekend is in the single digits.
you have a

around the TBI
and idle learn procedure that needs

engine off, then letting the ignition key on for a couple of minutes. This is what the installation procedure says to do after the new idle air control valve is installed.
You said there are no codes, was this done with a

shorting the pins on the obd connector.
If you are able to see live data, check the tps voltage

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