Completely forgot about my original post here. I eventually traced the problem to the idle speed screw. It was loose and the engine vibration caused it to rotate in or out. I could adjust it to a decent, low idle then drive the car for a few miles and the idle would be back to 2000+. Solved the problem by removing the screw and reinstalling with a stiff spring under the head. The spring tension eliminated the vibration-induced self-adjustment.
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