You might check to confirm that your car uses an auxiliary air or slide valve like other V8s. I think it does. These operate thermostatically to increase the air flow during cold starts. As the engine heats it closes off to reduce air flow so that the idle slows down. These valves can get sticky at times and the speed stays up if stuck open, or always stays low if stuck closed. It is at the top front of the engine, has a pair of large diameter rubber hoses running to and from it to the manifold. There is also a large brass slotted screw near by for adjusting the idle speed.

Search the forum, you'll find several posts on this.

FWIW,

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