Overdrive

I have a 95 jeep grand cherokee 6cyl. 4.0 4x4. Was driving up a grade and it was shifting fine. Started to make the decent back down the other side and the vehicle suddenly downshifted into 3rd while doing 70mph and the OD light came on indicating the OD was off. Within a day or two after that, my transmission started shifting eraticly, not shifting at times and finally not downshifting by itself. The shops diagnosis was that it needed to be rebuilt as well as the torque converter. I pulled the transmission out, had it rebuilt, then I put back together. It test drove fine until about 100 miles down the freeway (doing 60-70mph) the transmission suddenly shifts out of over drive into third again (same indication from the light) In the mornings it seems to return to normal until I drive 60+mph for longer than 6-10 miles. Help Please!