iPhone is a 3G iPhone 8GB bought outright from VFNZ store, running 2.2.1.

I'm an On Account customer, so my APN as advised by VFNZ support is "www.vodafone.net.nz"

However, when I configure the phone that way, periodically it gets bored and changes back to www.iphone-vodafone.net.nz

Sometimes it does it immediately and reliably on trying to initiate a data connection. Here's a video of the behaviour:

http://blip.tv/file/1835540/

Anyone know how I can convince the phone to stick to the correct settings?

VFNZ tell me the setting is in the handset software, which means it can probably be changed in some XML config file deep in its innards, but that requires some voodoo.

I hope it doesn't come to that, but this behaviour is pretty annoying, as the video above will show :)

If you know how to make the device behave, please let me know!!

Cheers