ROUGH IDLE

I recently bought a 96 Chrysler Concorde with 118,000 miles. It has a 3.5 engine. Idle is very rough and acts as if the engine will die, but as of yet has not. I have checked codes both with the on board computer and at a shop. No codes showing. I am at a loss because the car drives well down the road, gas mileage is good, and there is no smoke.