I have no answers, but want to provide information from my experience just in case it gives additional clues:
1. I have a Singapore phone. In general I update the firmware quite often, usually after the new one is posted on
[Stock Official Firmwares] I9300XXEMR2 (4.1.2) - Germany (VD2) [07-02-13] - xda-developers. I always use Mobile Odin.
2. I was on I9300XXELL5 / Germany without any problems.
3. I've upgraded I9300XXELLA / UK and the freezing started. I lived with it for few days (pulling battery, rebooting, uninstalling applications, changing settings)
4. I've upgraded to I9300XXELLC / Spain without any improvements.
5. I've downgraded back to I9300XXELL5 / Germany. The freezing DID NOT stop immediately. It stopped after few days. I was on this build for 4 weeks without any freezing.
6. A week ago I've upgraded to I9300XXEMA2 / Singapore. The freezing came immediately back. I lived with it for half a day and then downgraded back to I9300XXELL5 / Germany. Again the freezing didn't stop immediately. But it stopped after a day or so. Now again I have a freezing-free phone.
Now I'm stuck with I9300XXELL5 / Germany but very happy with no freezing. For those of you that have no other options try the same firmware, give it few days - and maybe you will have the same good results. You won't be on the latest, but you will have a usable phone.