[acidhax]

If you go to:

Settings -> Personal -> Phone -> Services -> Mode Of Operation
And select 3G Only or 2G Only

and re-enable standby.. and tell me if THAT helps.
I've read that it could be your SIM, and being in a weak 3G area could be causing it to halt.

OR alternatively if THAT doesn't work, if you know how to edit registry (CERegEditor):

HKLM\System\CurrentControlSet\Control\Power\State\ Suspend\{GUID}\Default

Change to 0?