The good news is that it's either the motor in the window regulator, the switch, or the wiring in the hinge area of the door. The bad news is that it's one of those 3 things. The easiest way to diagnose it is to leave the door trim panel off and keep a voltmeter in the van. When it acts up, check the electrical connector to the motor. If you're not getting power, then you've eliminated that part of the puzzle. Next, check the power feed to the master switch. If you're still not getting power, then your problem is a broken wire inside the hinge area. Best way to fix that is to pull the old wire out and thread in a new one. Make the splices in the kick panel area and inside the door--no splices in the hinge area.
If it turns out to be a regulator, here's an article on how to replace it.
Replace power window regulator