Guessing vacuum leak or a dirty IAC (idle air controller). A dirty IAC can be fixed by spraying at least 1/3 of a can of throttle body cleaner into the throttle body while the engine is idling. The engine will die when you get enough cleaner into the IAC but just restart it and spray some more. Raising the rpms will cause the cleaner to bypass the IAC. The IAC is what provides all of the engine's air when you're not stepping on the gas pedal. If the IAC is dirty it can stick and provide either too much or too little air causing too high or too low rpms.
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