AudiQ70615

I'm having an issue with my 2010 Q7 revving high when in low gear. It only does it when going up hill and have to switch to using manual mode to prevent it from happening. Not sure if the altitude has anything to do with it. Took it to Audi and was first told to flush trans. Took it back to have the trans flushed and was told that's not recommended and now being told its 2 of the cats. Any advice?