hi i have a 06 rebuilt 48re with a transgo shift kit and under light throtle 20% or less it shifts from 1st to 3rd a bit worse when tranny is up to temp. but anything over 20 % seems to shift some what normal also it will shift like this as slow as 20kmh and you have to go aprox 75% throtal to get it to drop back down.
any ideas

 

Have you done a flush on it recently? Sounds like gov. Solenoid and or transducer. If you haven't done a flush then when you drop the pan I would replace those. They are relatively cheap and easy to replace once the pan is dropped

 

tranny is rebuilt and 2nd transducer and gov. solenoid