When in closed loop mixture is controled by the system. The common idle problems are caused by lean misfires either due to vacuum leaks or injector flow restriction.
Because the system is controlled based on O2 sensor judgments of exhaust mixture and because the O2 sensor reads the richest cylinder, the mixture is too lean for any cylinder even slightly different in flow or air intake.
After fully warm try pressing down on the air flap. If the car instantly smooths out and then gets rough again it is running good as the lean cylinders are richened and then the correction mechanism leans it back out and it returns to its prior condition.
Bosch Master, ASE Master, L1
33 years MB technician