I’ve got a 2009 Dodge Charger R/T 5.7L with an automatic transmission. It a has tap shift option where you can shift the gears yourself to have more control and give you more power when accelerating and decelerating. It sets in my garage most of the time now and has 157,000. It runs great, on the highway the past weekend I was merging on and my cars tap shift wasn’t working, the car was on I-20 in Texas and sounded like it was stuck in 3rd gear. The RPM’s were high and not normal.
 I was able to make it to my exit and pull over and turn off the car and restart it and regain Normal operation of the tap shift and all 5 gears.
Is there a loose wire somewhere in the vehicle or a dirty sensor that the gear shift doesn’t recognize I’m trying to shift the gears and why would the transmission stick in 3rd gear. It may be time for a transmission fluid drain and refill. I’m aware your not supposed to flush the transmission fluid, is that correct. 
Thanks, Scotty. 

In 2013-2014 under warranty at around 50,000 Dodge did replace the transmission. It was in limp mode and wouldn’t leave 1st gear.