Definitely sounds like you have an issue with some slippage on your 6R80 A/T equipped Mustang. Doubt if you'll get many replies to the issue here on this forum. However, since all 15-18 model Mustangs equipped with the 6-speed A/T transmission utilize the same model (6R80) you can pretty much do a search of all the Mustang websites and see if there's someone out there having similar issues with the 6R80 in their GT or V6 model.
2016 Ecoboost Mustang
2005 Mustang V6 (Sold)
2003 Terminator (Sold)
2002 Mustang GT (Sold)
Last edited by Uncle Meat; 01-25-2018 at 10:21 AM.