I am not familiar with an actron scanner ... Does this scanner retrieve a "live data stream" that you can watch as the vehilce is operated?
IF the TPS has a bad spot in it the PCM will receive faulty information.
A view of the data stream will indicate IF the PCM is commanding OD while the trans drops out of gear. That would indicate an internal problem
The problem is IF the scanner shows all is OK with the data stream and the PCM commands.....
WHY is the trans dropping out of OD or is the Forward clutch pack the problem and it is releasing???
Or is the solenoid weak and not holding pressure for OD?
OR is the band apply pin not long enough and the servo can not apply it properly in OD? I've seen that many times.
Was the band a new unit during the rebuild?
Was band pin length checked during rebuild?
What does your trans rebuilder suggest as a process to move forward?
What is this "idle switch" that can be activated? I am not understanding that comment in a different response.