Cars Q&A

In the morning when the engine is cold, the car idles well for about 5 seconds with RMP steady. Then the RPM dips and idling becomes very slow and rough causing the cabin to vibrate (shaking the cabin). It can be best described as backfiring situation. Then, 20 seconds or so, the car goes back to normal. This is a 2010 300 SLK, and according to the computer of the service at the dealership, every thing is fine. Can you help?