My son and his girlfriend bought this 2013 Expedition Limited with the 5.4 3 valve from a really shady car lot. So within the first few days, he will be driving, and all of a sudden it downshifts and will not shift back up, and it will last until they drive home slowly, and then it will work fine for a day, then start acting up again. Someone told him, or he read somewhere, that it's the output speed sensor. I am only going by what he told me haven't had a chance to drive it myself yet. Does anyone have any input or ideas about this?

 

Suggest you have the vehicle checked for codes.
That should provide some idea of what is malfunctioning.and post the code(s) here.

If the problem is gone until the vehicle is driven again the PCM is getting reset every time the key is turned off.
Doesn't have to be a big problem , but the system is just letting you know there is a problem.