01 Venture Won't Start

My brother-in-law has a 2001 Chevy Venture. At times, it won't start. The battery has been checked, and a mechanic has tried to find the problem. Of course, when he has it, it always starts. The battery is good, and he checked the cable connections. He doesn't think it's the switch. Any ideas? Would a bad battery cable cause this? It sometimes will crank all the time for a week, then all of a sudden won't start. After a couple of hours, they try and it works.

Its not clear from your description, but I gather the van won't crank over when it won't start? Need to know what else goes on during key in crank position. Are there other lights working during the no-start condition? Need to know if the starter clicks or everything just goes dead, etc.
Often, a starter can have 'dead' spots where it won't move at all. Bumping it a bit with a wooden stick or rocking the car may help with that, but it could also be a bad cable to the starter, or even a bad key switch. You need to do more observing when this happens.