94 Mustang GT shifting problem

I have a 94 Mustang GT with an auto trans. After it warms up it starts randomly downshifting or seems to neutral out, (although I think it's actually trying to go into first gear at high speeds) all while cruising at constant speed. Trans was recently rebuilt. I've heard it's the Vehicle Speed Sensor from some people and that it's the Neutral Safety Switch from others. I think it's the Speed Sensor but I'm just guessing. Any input would be much appreciated.