Transmission shifts to often and never goes above 7th gear. Engine may stall when driving highway or in town when starting out from parking lot very dangerous to be in traffic .
As Yoda would say “warranty you have”.
Having said that, modern transmissions shift a lot.
How fast have you had it up to that it does not upshift when you let off the gas or cruising?
It shouldnt be stalling and it is probably still under warranty unless you have put over 36000 miles on it in 2 years . Take it to the dealer. You have the 850RE transmission and how it shifts depends on on how you are running it , whether in ECO mode , normal mode or Sport mode . ECO mode will make it shift sooner and get you to 8th gear sooner than the other two modes which may not make 8th gear until 55-60 mph .
It just may need a programming update.