Why does my vehicle s rpm idle below average?

I drive a 1998 Honda Civic LX with 105,000 miles. I gaved my car a full tune up last year, including a timing belt and water pump replacement… When I m at a stop sign or red light, my car s rpm drops low to where the whole vehicle starts shaking. It feels so low to where my vihecle will shut off. What s causing it?

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