Car turns off and keeps shutting off

Is the check engine light on, other lights lit up on the dash? Then scan for the trouble codes. Any other noises? Any recent maintenance?

My mechanic checked it he did a tune up he had it for about a week car didn’t do it then when I pick it up it up it did it two days later he said next step is to check every little thing

Someone told me it might be the computer

i wondered about torque converter clutch? It locks up at highway speed for better gas mileage then unlocks when you step on the brake or supposed to unlock. down shifting might cause it to unlock? If it's locked up when you come to a stop, it'll kill the engine. I'm not a transmission person. I might be barking up the wrong tree. good luck