transmission problem

bare with me please as i dont know proper terms for what im about to describe but i'll do my best to be as detailed as possible. so yesterday when i was driving i flipped the switch to go from 4th to 5th and when i went into neutral the transmission started making a constant clicking sort of grinding sound kind of like i was trying to go into too high of a gear and when i tried going into 5th it wouldnt go in i just got the same clicking of trying to go into too high of a gear. so i back down to 4th and try again and it goes in without a problem. also that same day i was climbing a hill in 4th gear loaded and weighing around 99,000 lbs when the trans felt and sounded like it popped out of gear but it didnt pop out the stick. so i dropped down to 3rd and the same thing and a soon as i got up the hill everything was fine. also when im backing up it also feels like the trans is skipping. its an eaton/fuller by the way.
our mechanic has replaced two air valves and what the parts guy called a puck on the trans but the problem persists and he's stumped. also on a side note its been in the negatives in terms of tempature and single digits during the day whether this has anything to do with the problem i dont know.
any ideas would be helpful but please only from people who have dealt with this problem or have a really good idea as to what the issue may be.