no third gear at wot

i just saw a post like this not to long ago but i couldn't find it. anyways, when at wot third gear disappears. just feels like hitting something solid, there is no gate to push the shifter into. why does this only happen in third at wot? what is the fix? if it is master/slave how hard is it to change out with jackstands and average tools?
edit: oh yeah, 94 Z, M6, 130,000mi.

ttt

anyone?