Something is toasted whether by heat, mileage or driving technique. I have heard that heat is what wears many slush boxes out. Perhaps a fluid change and a transmission cooler might bring it back for a while. Remove that grill block to see if that helps. If you are using your AC turn it off. Run the heater. What does your Scan gauge show? Any codes?
You have been making your transmission do things it was not designed for. ei: shifting into N and back to Drive. I am sure that the manufacturer did not anticipate owners doing that. Quit shifting your auto into N and back to D while rolling.
Looks like even mcrews expensive car must have a cooling issue with a grill block.
Everything is fixable. Keep telling yourself that you want to learn what the inside of your transmission looks like.
