I spoke with a technician about this before and his recommendation was that the oxygen sensor could be the problem. I apologize for the length of this, but wanted to make sure to get all of the information here. You probably do have a faulty oxygen sensor. With 91000 miles on your car, it would be good practice to replace it anyway. If your problem persists, then scan the computer for fault codes.
Oxygen sensors go out of range at times, but still work.