Miami 2005, his first big final. The first set of Wimbledon 2006, the only time I've seen him overawed by an occasion, but he got it together so well afterwards. The last set of Wimbledon 2007, much as it upsets Federer fans whenever I say it. These are the only ones that are coming to mind for me, where his performance notably dropped off a cliff in comparison with the rest of the match without physical factors coming into the equation.
I suppose you could say he took his eye off the ball in that third set breaker with Tsitsipas in Australia, but I'm not sure I would go so far as to call it a choke. He made a couple of mistakes, let his opponent back in the match, and the guy took the chances and stepped up.
Some of the matches listed here so far are making me LOL pretty big.