Car dies at random and starts back up after 15 to 30 minutes?

My car is a 2002 hyundai accent. It starts most the time fine but at random will stall out and I have to wait to have it re start again. It dies with no crazy sounds. I turn the key on the lights inside work. I tested the battery and looked and the terminals so that's not it. All I can think of is the alternator.