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Hello and thank you for trusting us with your question. I'm DJ.
There is a fast idle circuit built into the throttle body. It works like a thermostat. When the coolant is cold, the valve opens up and allows more air to bypass the throttle butterfly. This raises the idle speed. When it heats up, it closes and brings the idle back down.
It's the most likely cause for this.
Let me see if I can find you a picture.
Scroll down a ways and you'll see how the valve works.
Normally it's just gummed up.