When idling, or driving slow, the engine cuts out and won't restart.

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While driving the car and waiting at a light, the engine died. No hesitation or sputtering, it just stopped. After sitting 10-20 minutes, the engine starts back up. It did this 6 times in a 90 minute time frame. Mechanic had the vehicle and was unable to duplicate, the car was driven 500 miles without an incident, and it started doing it again. I know it sounds crazy, but the only thing I can think that has been the same is I've had "automatic" selected on the climate control both times. Without it, I haven't had the issue now for 2 weeks of in town driving.