I had the same problem for about 3 months. A few trips to the dealership and they finally settled on a bad O2 sensor. I didn't like their service fee so I decided to replace it myself. Fortunately, I had to take it in for a TPMS reset and when they gave it back - no more O2 sensor problem.

I think they did enough resets of the ECM that whatever was out of tolerance was recalibrated.