downshift on releasing throttle? - you mean upshift?
if you are losing O/D then you might want to check the 2 pin wiring connector on the transmission for bad wiring (if this is the oldschool hydraulic box with pressure cable and electric O/D lockout)
the wiring is basically going to the O/D solenoid. When you apply power to it the transmission will "shift out of" O/D and continue in 3rd gear (direct drive). This shift pattern is also affected by the pressure cable setting.
If you are facing extremely delayed shifting in all gears then the speed governer maybe stuck with some debris/trash in it and is held up. Its removable from the diff casing on the transaxle, its right above the left driveaxle cup behind a metal plate.
When you changed the ATF, did you flush it out or just drained and filled about 3 litres? - the transmission holds close to 8 litres.