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The best way to find the problem is to run the codes. You can buy a code reader at your local auto parts store for around $20 - much cheaper than just replacing parts until you find the problem. Hook it up to the connector on the wiring harness and run the key on, engine off and the engine running tests. If you have a bad sensor or solenoid somewhere, you will get a trouble code.
From your symptoms my guess would be a sticking idle control motor (the cylinder that bolts to the side of the throttle body).
Pete
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