I had a TH 350 that did similar. When cold it would go into reverse, & forward fine. Once it got hot id had no revers unless you put in a forward gear, then as soon as it moved go directly into reverse. It would back fine then, no slipping at all. Turns out on the rebuild there are two snap rings that lock onto them selves on the pump. The end broke off of the one for reverse, causing to not have fluid pressure to the trans from the pump. When shifting it from forward the fluid pressure didn't drop very much and would stay up enough to power reverse.