High idle speed: '94 T-Bird with V6 3.8L

My '94 Thunderbird (3.8L V6) has a problem with idle speed -- once the engine is warm, and in neutral, it will idle anywhere between about 1500 rpm and 2000 rpm. Idle speed in D is normal (700-800rpm).
Any ideas what might be wrong and how to fix it? It seemed to suddenly start doing this a while ago, although earlier, the idle speed (while in D) was perhaps a little low (and the revs would not be steady).
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Whoever wrote:

IAC valve, most likely.
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