After driving car for 20 min, putting in reverse car stalls, stall at stop light

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Drive car for at least 15 min - not when it just turns on and idles - the car will stall when trying to reverse and now it stalls after coming to complete stop at stop light. Happens frequently now - shop says it is not throwing any codes. First time it happened the dash computer lights where mileage is went wacko - when car is cold it will go reverse no problem.