The old transmission was probably fine....these trans need a lot of fine tuning before you can say its the trans fault...

If the car isn't running up to power you will have shifting issues....if the transmount is bad you will have shifing issues..if you have vacuum leaks you will have issues...

you must make sure you car is fully maintained and it is producing the correct power...this includes

adjusting all linkages to the fsm guide
confirm no fuel leaks, clogged filters, clogged tank screen
clean air filter
oil change
valves adjusted
turbo maintained
rack damper bolt
pressure relief valve

then you must replace worn out bushings
trans mount
engine mounts
shiftier bushings
throttle pivot bushing

an diff oil change can also help.

then you must go to the vacuum system
you must not have any vacuum leaks
check modulator

then if your car is equipped
adjust the bowden cable

you must also confirm the kick down switch and solenoid is working properly

all this stuff is very very time consuming but if you want a proper shifting transmission it must ALL be gone through..

__________________