I had my friend and his mechanic step father put in a new crank shaft and pistons in my 1998 3.8L v6 automatic and there’s something wrong. When I first back up out of my driveway and head off the car has a hard time catching gears. After it’s gotten through the gears once however it seems to do ok. What could this be?

 

you said your car has trouble shifting but after it's actually made it through all the gears it shifts fine?