It may have the std wiring fault on these, look VERY carefully at the connection that goies from the iggy switch to the solenoid on the starter, these sometimes snap/break at the terminal, people crimp new ones on, but they don't often make good enough contact, and so don't hold the solenoid strongly enough in the start position, I've done a van recently with this very problem, that had a new battery and starter, before being looked at by myself.
I cut back the wire and soldered a new terminal on, and heatshrinked it with adhesive lined heatshrink, important as solder joints don't flex at all, they just break, the heat shrink helps massively here, worth a go, I assume they used the tdi ecu as well >
tabetha