I never had it go out at those types of speeds, but I did have it stall several times at slower speeds. In my case it was the injector. After I had it replaced I had no more problems. Don't know if that is your issue but it sure sounds similar. I have the exact same year model 2500. The mechanic also told me to remove the cover that goes over the top of the engine. He said it was purely cosmetic and only trapped heat.
Good luck,
Random