d6h series ii

have a 1989 d6h and the darn thing works perfect for an hour or more but after that the trans temperature get real hot. it pushes great till oil heats then things start to happen . when your switching from forward to reverse sometimes you put leaver in gear and it will do nothing? we have changed transmission filter and cleaned screens and the screen was full of rust but cleaned put together but no difference.

d6h series ii