I've got an '83 Z/28 with LU5 and 700R4. Previous owner says that trans was rebuilt about 2K miles ago with a "Corvette servo".

At anything over half throttle to WOT, this tranny shifts violently hard from 1st to second. It also won't downshift when I'm in high gear and give it throttle. I've already set the TV cable. Otherwise, it seems to perform normally.

What should I be checking?

 

sounds like the previous owners statement of "trans was rebuilt about 2k miles ago with a corvette servo" actually meant "installed b&m shift kit"

if your cable is adjusted correctly and it still wont down shift, it may have that piece in the trans that makes 4-3 down shift only work at like 75% throttle, i forget what the thing is called, but i have it on my trans, and i like it, keeps the trans from downshifting while cruising at higher speeds, or it could be a problem with the trans. but i do know if you got the b&m shift kit in your trans, you are going to be buying a new transmission sooner than you would like.