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When this happens, does it feel like it drops down a gear briefly, or more that the engine is surging
How many miles on it? How many miles since last transmission fluid service?
Please remember that I am not there and therefore rely on you for as much detail as possible and on occasion may need to ask for more information
Surging is more an issue with the engine but because it doesnt do it in manual mode, that makes it transmission control system. I am not sure what this will be and will opt out to see if another expert has an idea
It sounds to me like the circuit for the torque converter clutch is kicking on and off. It should set a fault if it is the electrical portion of the control circuit.