96 4t60e shifting problem

Hi, I need help trying to figure out my problem. I just installed a rebuilt trans, and it shifts fine but it will drop out of overdrive with the slighest throttle reduction, since I live in the hills it makes it extremely hard to drive with it shifting all the time. Should it not hold about 5-6 mph before unlocking? Mabe reflash the computer, bad map sensor, throttle position, speed sensor ?
Thanks, Dave

Scan it and look at the tps inputs also you may have to get a trans function scanner on it and look at what the pcm and tcc solenoid are doing when this happens.

Thanks, Dave