nevermind..

 

Fortunately you have the S with the auto, (not the CVT in the Std cooper) it is generally well regarded when it comes to reliability. Since it will function well and not slip in manual mode, that is a good sign that all is not lost. Not sure what the problem is, but a fluid check, and maybe a flush may be needed. I'm sure someone will chime in. Since the transmission is very much a regular auto transmission from what I understand, a phone call to a local transmission shop might be great advise if you don't get a good answer here quickly.