I've had this problem in my Stang for a couple of years but it's just resurfaced because I haven't done much highway driving in quite a while. I have a 2000 GT (T45 trans).
Under normal conditions I can run it up to fifth and then back down again no problem. Once it's been on the highway for a while (30-45 mins) it won't let me downshift to fourth, it just grinds. If I slow down enough to shift into third, and then go back up to fourth, it still grinds. The only way I can get it back into fourth gear is by coming to a complete stop. After that, I can upshift normally, but as soon as I hit fifth, it'll grind the downshift to fourth again. Any ideas?
Under normal conditions I can run it up to fifth and then back down again no problem. Once it's been on the highway for a while (30-45 mins) it won't let me downshift to fourth, it just grinds. If I slow down enough to shift into third, and then go back up to fourth, it still grinds. The only way I can get it back into fourth gear is by coming to a complete stop. After that, I can upshift normally, but as soon as I hit fifth, it'll grind the downshift to fourth again. Any ideas?