All the debate aside, this is why GOD invented the warranty. Let the dealer figure this out.
For the sake of argument, a vacuum leak would result in rough idle as well as poor performance. If the leak was at the master cylinder, it could hide until you step on the brake. You might prove that one by stepping hard on the brakes while in gear and stopped.
Sounds like one to me.Make and model are a good start. I'm thinking vacuum leak - a big one. How is your gas mileage compared to norm?