[CDMA] Cannot program MDN/MSID?

[Gunpuppy]

Heya fine folks at PPC Geeks!

I'm having a little issue with my friend's Diamond. I'm sending my own Touch Pro for repairs, and he's lent me his "old" Diamond in the meantime (He got a Blackberry Storm recently). My provider has changed the ESN on it's end, so I know it's good.
However, I seem unable to program the phone at all. I get into the EPST using the master lock code and change the MDN/MSID fields. However, every time I hit the "yes" at the "Update Items?" dialog box, the phone turns off and doesn't restart on it's own (From what I understand, this shouldn't be).
Once the phone's turned back on, it's back to the old MDN/MSID (5141111111) and my own isn't present anymore.

I've tried various ways to get to change this:
##[lock code]#
##778# + lock code
etc...

At this point, I'm frankly at a loss.

Also, I do not have the stock ROM installed. Could this be an issue?

If anyone has any idea what I could do at this point, I'd be deeply grateful as I don't know what to do.

Thanks to all in advance.