You are using normally the car... than the transmission start getting crazy. The car runs but the gear changes looks as a old lady on a manual transmission. If you shut the car off and after 10 seconds turn it on, the problem disappear. It appears again after 15 minutes of running engine. Why the car runs normally, change gears smoothly but after 15 minutes it gets into some safety mode? I don't see any need for changing the whole transmission, but one mechanic and the dealer do.