I was here a few weeks ago with this problem but I can't find my thread.
I have an 8320, with Orange and 4.5.0.52 OS
As far as I can tell, what happens is that my phone purges all its text messages every week, for no good reason! Except the saved ones...
I really don't know how to fix this and it's really annoying.
I'm listing all my settings below:
SMS and MMS > Options > General Options:
Actions
Make PIN Messages Level 1: Yes
Auto More: Yes
Confirm Delete: Yes
Confirm Mark Prior Open: Yes Keep Messages: Forever
Options > SMS:
Delivery Report: Off
Validity Period: Max
Sent as: Text
Data Coding: 7 bit
Disable AutoText: No
Leave Messages on SIM card: No
Network to send over: CS
Number of Previous Items: 20
Retries: 5