engine not returning to idle 3.0l pushrod engine not HO model with automatic tra

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I am having an intermitant problem with engine not returning to idle on hot or cold start up also when driving. When starting engine it occasionally races above normal warm up hi speed idle. This can continue 10-30 seconds than return to idle. Also when driving take foot off gas car continues to travel @ same rate of speed as if foot still on gas. Example, I accelerate to 30mph take foot off gas car continues to travel @ 30mph. for some time than returnes to idle on it's own. If I shift into neutral engine returnes to idle speed. This sequence of events may repeat itself many times during my journey or happens only once or twice. I replaced idle air control valve with an Autozone valve & made problem worse. Now engine speed too high to shift into drive. What is causing this problem? Cruise control is disconnected