After my daily 20 mile / 30 min highway drive home in the evening in my 2013 S7, I sometimes find it difficult to get the car to reverse. I can shift into R no problem, and sometimes the car even starts to move normally, but it will eventually slow down and become unresponsive. I've given it some gas and it feels like it's trying to move but getting resistance. Usually I can shift it to D then back to R and it works fine. There are no loud noises, smoke, or leaking fluid. This never happens after a cold start, as I back my car out every morning without issue.

Periodically when the car is running but standing still, I can smell something from inside that closely resembles the smell of natural gas. Not sure if this is transmission related or because I just switched to 93 octane from 91, cold turkey.

This has been happening for about a month and so far hasn't gotten worse...