Some 2011 Ford Fiesta models may exhibit a rolling idle when idling in drive with the air conditioning on.
According to Ford, the condition usually occurs when the ambient temperature is above 80 degrees and/or when driving at altitudes above 8,000 feet, and is due to software anomalies in the PCM. Reprogramming the module with updated software is the correct fix. Check with a dealer for the application specifics of the flash.