If the quality of engine idle fluctuates with engine temperature it could be an engine sensor that is acting up when it is exposed to heat affecting the ignition or fuel control. I looked to see if there are any recalls, there is one for the fuel pressure regulator but fro a problem different than your. I wonder if it could be a vacuum leak? Even if the Check Engine light is not illuminated a good scantool that can display live data with the engine running will show air fuel ratio, fuel trim figures and instantly read what the engine sensors are telling the computer in your car. Even if the Check Engine Light is not on pending codes may be stored in the computer in your car indicating where the problem may be.