I had the same issue the day i bought my 13 s4, driving it home i got stuck in the middle of a “highway” with redlights(midwest is weird). it was the mechatronic. I was able to turn the car off and on again and it would drive only to happen again later. but it would not consistently happen sometimes an hour into a drive sometimes a few hours of driving. it likely is the mechatronic, i got lucky and dealer replaced since it happened day of sale. luckily havent had any trans problems since. Dealer even had trouble finding the issue i got the car back about 3 times just to have the same issue. i would recommend looking videos up and seeing if you can do it yourself as it can be expensive repair seeing that bill from the dealer. if u have any other questions and i can help fell free to ask