Its most likely you ISC. When the pedal is all the way up, the throttle plate is all the way closed, and the engine idles. The idle switch is triggered by a small lever connected to the plate inside the throttle body that regulates the amount of air entering the engine. When the idle switch is open, the throttle plate is at least partially open - when the switch is closed, the throttle plate is closed. Your idle switch is acting up, causing it to open when you dont have your foot on the throttle.
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