Re: over shifting transmission
Welcome to the garage.
Did you check the fluid level first? And smell the fluid for if its not burnt?
If this is ok, i would check for any fault code set (with a scanner).
The shifts are based on a pre-programmed schedule and the input from input and output speed sensors and the TPS. If you have a scanner which shows live data, then you could check the TPS. Just press the accelerator and see if the % goes up gradually without spikes.
Also, are you sure the transmission is shifting, or is it the torque converter clutch which is releasing and applying? The scan most probably will tell more.