96' Taurus Transmission overdrive

Hi I have a 1996 Ford taurus 3.0 it will shift into overdrive the first time I start out then when it gets 5-6 miles down the road it will kick down and will not shift back into to od till it sits. I checked the PCM or any codes all that came up was it was not in Neutral or park. the transmission was rebuilt about 8 mo. ago because that little spring broke for the overdrive.
I am wondering if anyone knows if it is the PCM, TR sensor or the transmission itself.