New or rebuild the transmission?

I got a 85 firebird, v6... i need to handle my . but should they fix it, or should i buy a rebuilt one, or new? i just want it to ride good. its automatic, but sometimes it takes off in the wrong gear, like a hard-loud noise. doesnt slip too much like ur typical slip feel. but its like a lil gas/brake/gas/brake but suttle motion... not like hard brake but like let ur foot on and off the gas, lite, you know what i mean? i put new t-fluid in, but idk what to do