The AT light turns on, and the car downshifts temporarily.
There are many things that can cause a condition like the one you are describing. Unfortunately, with the limited information provided and all the variables, a diagnosis is difficult. Low line pressure, faulty SERVO, bad shift solenoid, faulty pressure control solenoid, just to name a few are some of the things that can cause this type of symptom. A proper test of all transmission sensors, solenoids and circuits is necessary to properly diagnose this type of issue. A qualified transmission mechanic can perform these tests for you and suggest an appropriate repair.
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