1992 ford bronco transmission problems ....thin oil ? need rebuilding ? please help...thanks?

i just bought a FULL SIZE 4X4 automatic trans bronco 170k miles with a 5.0 engine ....the engine seems good and the transmission works good while cold .....when i start the truck and drive everything seems good and when it gets hot (i think )the does not grab any gear at all not even reverse .....it seems the oil thins out ....i dont think its a bad becouse it shifts good when it works and does work at all when warm......i have read not to change the oil becouse of the high mile .............................WHAT CAN THE PROBLEM BE ........