I am stopped at a traffic light, or waiting to turn it stalls suddenly

When I first start the car, no matter how cold, I can let it idle in 'park' for an extended period of time and it NEVER dies. The minute I drive it..be it a short distance (i.e. just to the post office), if I am stopped at a traffic light, or waiting to turn (in other words, idling "in gear") is when it dies. This happens whether the car has JUST been started, or after I drive it 72 miles to Lewiston and am at a light. Thought it was weird it wouldn't die, sitting in park, in the first place. Just had a tuneup and fuel pump replaced, so not that. When it dies, it dies cold...no chugging or anything...and, so far, will immediately restart without issue or effort. Still, unnerving for this to happen, albeit intermittently (of course), at a busy intersection.

Thanks for anyone's advice.