Having transmission problems

I have an 05 also with about 312000 miles on it. It is a 5 speed and I have had problems with 5th gear whenever it is hot outside. As the transmission heats up it will grind while putting it in reverse. If I drive long distances 5th gear will eventually act up. It seems I am getting it into 5th but when I release the clutch the engine races and the RPMs ramp. Usually by the next day it is operating normally once again. Til now I have not had 5th gear for 24 hours. The gear oil is fine. And there is a possibility of a bad rear motor mount all this being said. I really don't want to have to buy a new transmission, if I don't have to, does any of this sound familiar to anyone, and if it does, what am I dealing with?