fORD 250 4X4, auto transmission drops out of gear randomly,

Auto transmission drops out of gear randomly at higher speeds or has trouble shifting up esp. from 2nd to 3rd and 3rd to 4th. This may happen at anytime. It may go a whole day with out doing it then once the car is warm it will happen all day. It doesn’t ussally start until the car is warmed up. any ideas?

Also, on the advice of a mechanic we changed out the old trany and electronics, and it is still doing it.

Has anyone scanned the computer for codes???