This type of problem comes up often enough that I built a wiring bypass
tool to help track it down. The tool is meant more for situations where
the van dies and won't start again right away or ever. The point of the
bypass tool is to quickly eliminate most of the known wiring faults that
happen to 2.1 wbx and keep it from running. The ignition switch will be
used for cranking the starter but turning the key off won't stop the
engine with the bypass tool in use. This lets you eliminate the ignition
switch as a cause of your problem. If you fully install the tool and the
problem doesn't happen anymore then you know that most of the fuel
injection, ignition, and fuel supply systems are working and you won't
waste time and money chasing guesses for what part might be bad. Then
you can remove the 3 bypass branches one at a time to find which branch
has the problem.

http://www.thesamba.com/vw/forum/viewtopic.php?t=349429

Mark