Transmission Problems when Cold

[Ohiozr2]

I have a 2015 Sierra (5.3l 6spd) that isn’t shifting correctly after sitting over night. After driving about half a mile at 40mph, I will stop and turn to accelerate to 55mph. When the transmission is trying to shift from 2nd-3rd, it will “pause” for about 5 seconds and won’t accelarate nearly at all. This is in temperatures typically less than 40f after the truck has been sitting over night. Is there a TSB that addresses this specifically? I plan on taking it to the dealer as it is still under powertrain, however, I would like to go with as much information as possible. 

 

Also, has anybody had a transmission shop or similar flushed their transmission with fluid they provided? I’d like to use the best fluid possible, typically use amsoil.