Sounds like somebody's strapped a bomb to the underneath of your van, and they're running the trigger off the ignition switch. Do you have any powerful enemies?
serious answer is that usually this problem is caused by a faulty ignition switch or duff wiring from the switch to the starter motor. People tend to assume that it's the fault of the starter motor itself but these things are pretty robust, whereas the starter circuit isn't.
Starter motor has two feeds, one big, one small. You can use the big one to make sure that the starter is OK. Take the van out of gear, jack it up, and bridge the two terminals on the starter with a big old screwdriver. Should turn the engine over, indicating that it's wiring or switch.
The fact that the van started when jumped indicates that it's likely a crappy connection somewhere rather than the switch, but you can't rule that out. Try supplying power to position 50 on your ignition switch (thick red wire with a black stripe). THEN you can rule out the ignition switch, and if the starter doesn't run, look at the connections on the starter, the connector in the circuit between starter and ignition switch, etc.
Of course, first off you should probably check the earth leads on the gearbox and the battery, and the big chunky power lead to the starter.