engine shutters when down shifting, between 1500 and 2500 rpms, fuel mileage reduced.
Torque converter shudder? Doesn’t your vehicle have a 5/50000 powertrain warranty?
Is the check engine light on? What are the error codes?
If this car is still covered by that Powertrain Warranty, the OP doesn’t need to try to diagnose this himself, and should simply take it to the dealership.