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It sounds like you might have a vacuum leak. When the engine is idling, it's supposed to be running at a steady RPM. If there's a vacuum leak, it will cause the engine to speed up and slow down as the air leaks in and out.

One way to test for a vacuum leak is to spray some soapy water around the intake manifold and see if any bubbles form. If there are any bubbles, that means there's a leak somewhere and you'll need to track it down and fix it.

Another thing you can do is check the idle mixture screws to make sure they're set correctly.
 
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