Dash meter showing AT signal and car goes lamb

For recently few days I am facing a big problem which is AT signal !

my car suddenly shows AT signal when slowing down or coming to a stop and car make a high jerking and speed done both ahead and reverse


here mechanics are not very expert about Mazda. They are telling me to change the Transmission Box directly

but I can feel that no need to change transmission box cause I can easily hit 160 ~180 kmph speed

and the At signal not showing in running high speed