There are valves in transmission are made of rubber. They worn and harden over the time. So they did not provide enough seal when they are cold, after warmed up, it softed and expanded a little. The problem go away.
It could not be fixed cheaply, you need to replace tran for it.
You could replace the tran oil, and add little additives which would soft the rubber. It should buy you some time. But this car is at end of its service life.