Toyota supra transmison issues?

About 7 months ago i bought my supra. Its a 1988 non turbo 3.0L, i bought it naive knowing it had transmission issues, so i didnt test drive it. My issue is that the transmission slips between 2nd and 3rd. When i first bought it i flushed the tranny and it was fine for about 1 weeks when it suddenly slipped. I got it to shift and went to take it home, and the tranny basically shut off. It acted as tho it was in park the rpm gauge would climb but it wouldnt move. I shut off the car in a parking lot and much to my dismay turned it back on cause i had no option but t drive it home. It did not slip again so i deduced it maybe electronic. I brought it into a tranny shop and they found the speed sensor to be bad so that was replaced. I took it out and drove it about 50 miles and it slipped again. The overdrive off light never completely turns off even when it is engaged it just flashes at me, does anyone know what might be causing this?