Every quarterback goes through stretches when they've been out of sync. Even Mahomes, who, as johnmd20 points out, is at the top of the food chain. During the first LV game this year he went from the 9:04 mark of the second quarter to late in the fourth quarter, when the offense only put up 3 points, and Mahomes went 9-23 (39.1%), for 108 yards, with an interception and 2 sacks. It happens. Wilson, Rodgers, Brees, Jackson - they've all had crappy stretches this year. Even in Brady's best years, he's always had a game or two where he was bad. In their 16-0 season, Brady had a 3 game out of 4 stretch where he had a passer rating of less than 80.
He's playing like most good QBs do - a bunch of tremendous games, a few crappers, and an overall good season.