It sounds like to me that the clucth disc has worn enough now to not allow full disengagment due to noraml wear, try adjusting it again and see if that fixes the down shifting problems I've onw and used a bunch of Mopar 4 speed NP833 that had ATF in them from Ma Mopar and never had a issue with wear or destructions, that was in the Mojave Desert in SO CA with ambient outside air temps above 100 F most of the days in the spring, summer and part of fall