Fred,

Sounds like you are getting pretty good at trouble shooting. Sometimes it just takes time to sit down and think it through. Do you have a Jake Brake and a transmission retarder or just the trans retarder? If you have the Jake Brake did you try using that? I have found that ging down hill the Jake Brake has done a great job. I occasionally have to manually downshift to 5th to hold my speed with an occasional stab on the foot brake. As you have discovered, the retarder really heats up the trans fluid, and that is not a good thing.

Hope you get good news at Allison.