Mitch has to work on his halftime adjustments, anticipating the other coach's halftime adjustments, and accounting for what the other team is going to do going into the third quarter. The team is always terrible in every aspect of the game for most of the third quarter. Too many times the Spurs are completely lacking an identity or even a discernible plan of action on offense, which is when you see Dev or others chucking bad shots and making unnecessarily risky passes. Turnovers and Effective Turnovers (shots so bad you might as well just hand the ball to the other team) are what swing the momentum so quickly against the Spurs, which means that on defense you're spending too much time defending fastbreaks (even harder to do without Castle) and even when there isn't a fast break the Spurs are overhelping so much that the opponent is basically guaranteed to find wide open perimeter shots. This is why, even in a league where a 15-point lead is the new 10-point lead, when it comes to the Spurs it truly feels like no lead is ever safe. They could be up 25 points at halftime, and it's still possible for the game to go down to the wire. There are several wins where the game was much closer than the final score would indicate, because the Spurs didn't really put them away until about two minutes left, after multiple lead changes, the Spurs go on like a 9-0 or 11-0 run or something like that.
Some of these things will be easier to deal with when Wemby, Harper, and Castle are back, but it will be very evident against elite opponents even if the Spurs are at full strength. This is something Mitch needs to improve, and after seeing many games this season I don't think it can be fixed by experimenting with rotations alone.