Why is my trans shifting down and staying in third gear under no acceleration

What is your question?

When driving 45+ mph the trans will shift rather harshly down into third gear and never shift out, even after stopping it stays in third gear.I have to stop and turn off the engine for about 5 minutes or so for it to recover then if I go over 45mph it starts over again. Fluid is fine and no other problems exist. This has been happening on a more consistent level.It only happens after engine is at operating temp. Thanks