tranny dont shift right at all.

so i have a 1981 chevy with a turbo 350 and i just put a tranny in it.well when i i shift from second to 3rd it seems to shift fine for like a second then it just bogs way down. and wont it kick it self backout unless i get going like 60. could this be a kickdown cable problem because i dont think its adjusted right. and when your slowing dowing it jugs bad when its trying to downshift itself. its hard to explain i guess what it does its really weird idk! any ideas will be helpfull. thanks!!!!!!!

zack