buick tranny problem

i have a 1978 buick regel and the tranny is acting up it will work great till the car heats up then it will slip and make a noise i can seem to figer this out on my own any suggestions on trouble shooting this issue??

sure sounds like what happened to one of my cars,,,but the trans cooler adapter was not drilled through ,so no cooling of the fluid

trans cooler isn't cooling,, plugged line or air in cooler??
just a possibility