I have a 94 Z28. When it's cold it doesn't shift. It has to be warmed for about 5 minutes. The tranny is fairly new, and when checked was told it was fine. You can hear almost like linkage noise when it's not shifting. Not sure what that is, but nothing comes up on the computer. We have already had the mapping module changed, new distributor installed (2nd time) and tranny rebuilt. What'e left? Car is all stock.

 

Often when a rebuild is done, many shops just replace clutches and seals and call it done. My hunch is you have a sticky valve in your valve-body that frees up when the engine warms up. The rebuilder no-doubt ignored the valve-body altogether as they often do.

The other possibility is a stuck or misadjusted throttle valve/cable, or a faulty speed sensor (some GMs use different sensors for the speedo and tranny).