I have a 2000 Ford Focus station wagon with the 2.0 liter and automatic transmission. Lately right as it shifts into overdrive the transmission starts to studder. The fluid is fine and the tranmission shifts great in all other gears. It only studders right after it shifts into overdrive. Does anyone know if this is a sensor or something to do with overdrive lockup??? No lights have come on or anything. Any information would be greatly appreciated! Thank You!!