transmission or rearend

i drive a 93 Z28 camaro and ive had the transmission rebuilt 3 times. its an automatic. it seems like its not shifting as hard as it was when i first got it it has a shift improver kit. the other night when i was driven i got on it and it kinda tried to cut out or slip it had never done that before. im not sure what it is . do yal think it would be the transmission again or the rearend. ??????

cut out/slip it as in the car got loose (rear wheels lost traction and the rear end tried to drive out from behind the car, like fish tailing)? if that happened there's nothing wrong with the car, its supposed to do that.

well i can tell u i no what my car can do. im plenty sure of that . my car was kinda shaken when i was hittin the gas. it seems like it shifts gears to fast. do u think the shift improver kit needs to be replaced ???????

it dont seem to run as good as it did a couple months ago.