i just rebuilt my first c4 trans now i need help

ok i just rebuilt my trans and every thing is good it was not all that hard it was about 1 month ago and i have it idled varey low and when i sit at a stop light for some time like 3 min it will engage hard but it onley in engages at 1000rpms and i have it idled al like 600 to 500 rpms

 

i know seems simple but try checking the fluid level

 

i did its good i think i need to put a new trans cooler on it

 

Did you check the fluid level with the engine idling and the transmission in park?
A new cooler won't affect the shifting unless the cooler is leaking and you are low on fluid.

 

Sounds like low pump pressure. Tell us what was done during the rebuild. Did you replace the pump sealing rings? Also, is the converter stock or aftermarket?