2006 Dodge Charger RT Keeps Turning Off (help?)

So for a while now, my charger has been randomly turning off when I come to a stop light. I had it towed to a mechanic, but by the time he got it, the check engine was off, and it wouldn’t turn off on him. He had it for like 2 days, but not once did it turn off on him. The alternator is brand new, so I doubt that’s the problem.