Here's a bit of trivia for you...
First adjust the shifter, adjust it so that when the shifter is in 3rd gear, the trans is in 3rd gear and there is NO pull either way to go away from 3rd.
Pull the pan off this thing, look at the manual valve in the valve body when the shifter is in 3rd gear. The manual valve should be protruding from the back of the VB
Some shift kit instructions say flush, flush WILL cause you a problem.
The difference between 3rd gear, and partially exhausting fluid into the neutral passages is this 1/16".
It is also possible you have a full manual VB