Hauling a 30' gooseneck trailer weighing about 16k lbs. Cruising about 65-70MPH you start to come upon a hill. Truck starts to slow, drops from 6th to 5th automatically and maintains 65MPH. As the hill increases truck starts to slow eventually dropping to 45-50MPH and never dropping to 4th gear.

If on the same hill I see I'm slowing I can slap the shifter button and drop to 4th manually and truck speeds back up and holds a good speed all the way up the hill no issues.

This issue has been occurring for the last 2 years. Noticeably more regular the last 6 months and nearly every couple days. Of course it is the kind of thing that the owner notices and is able to repeat but not so likely the technician. The transmission doesn't down shift from 6th down to 5th early enough and I have actually noticed it from 5th to 4th also. The lugging is very annoying.