ive been having a bit of a pickle my old motor on my prado 3 door 2004 was a 3rzfe was totalled when i spun a rod bearing i was unable to find any parts for it so i was forced on switching to a new motor and transmission, 5vzfe automatic jap import, i made the switch the mechanic who worked on it did the conversion, however everything was fine, i changed the tbelt, plugs cleaned the fuelsystem tranny fluid change made sure that everything was on point.

heres my problem
when i start the car and drive it, it shifts early on the second gear around 1000-1200 rpms thats if im driving a slow pace, however when im driving normally with the rpms at a higher range such a problem does not occur.

would you be able to guide me on to what might be the problem ?

ill be more than thankful for your help

 

when it shifts early it feels like the car is losing power, the closest thing to it would be like on a manual transmission where you shift from 1st to 4th gear