03 Grand Caravan Ignition Problem???

My wife's 2003 Grand Caravan is experiencing some sort of ignition problem...so I think?? Sometimes when we turn off the van after driving around town, we turn the ignition and it won't start. It does not click or make any sounds. The lights on the dash turn on but no start-up. I've had it towed to the dealership twice and they have always been able to turn the ignition and it starts right up. They've done nothing to it on the premise that they were not able to duplicate the problem. Can anyone help? Any clue as to what it may be? Has anyone experienced this problem. The service department at the dealership says that they've check the grounding, starter, battery, fuses, and cables but have not been able to find the problem. Any clues.