I have an 88 Camaro that has a 383sbc and TH350 in it. I'm trying to get it running right so I can sell it. The trannie has never seemed to shift quite right. And once I got the engine running right the trannie blew. I had it rebuilt with Transgo shift kit while it was out of the car. Therefore I did the reinstall instead of the shop. Originally the detent cable wasn't even hooked up. It was snipped in half. So I bought a new one and attached it to the throttle bracket. It also has a stock Corvette torque converter.

Here's the problem: The trannie won't shift back down once it's in 3rd gear unless speeds drop to almost a full stop. Also under full throttle, it shifts from 1st directly into 3rd at around 5,000 RPM. Under half (or less) throttle it shifts 1,2,3 just like it should. Shifts are always nice and firm too.

The car seems to have plenty of vacuum and no vacuum leaks. It shifts the same with the modulator screw all the way tight or way loose. It shifts the same with the detent cable all the way pulled out and all the way pushed in.

The detent cable gets pulled out when the gas pedal is pushed right?

What else should I check?