Why is my Saturn shaking at idle?

I have a 2001 Saturn SC1 with 60,000 miles. When the car is at idle, it shakes and the rpm sits around 0.8. If I turn on the air conditioning, the vibration decreases greatly. Gas mileage is down slightly (26 mph vs 29 mph). I have had the spark plugs and wires replaced, but this didn’t help. Any ideas?

Intake cleaning would be a great place to start.