2003 Subaru Outback transmission modual

The transmission doesn't want to shilft into overdrive which cause either it to shift hard into the overdrive or the oil temp light comes on. They replaced the sensor that tells the transmission to shift and it worked the first time and started to work on the second time but after a week it stopped and has been acting up since. If we pull over and shift the car into park it resets the transmission and shift ok then. What causes it to act up the most is when it down shifts below 40-45 rmps. Do you have any ideas.