OK, go to this page on Sammobile
http://www.sammobile.com/firmwares/ and type into the search box (where it says "search device here") i9300o2u.
You need to download the first file that comes at the top of the list.
It is a complete firmware, modem and bootloader file so will update the entire phone os (if the option in Odin is available, tick "bootloader update").
After flashing reboot the phone and skip past the start up options, shut down and do the factory reset again then start up the phone and setup the owner options. After completing these steps, leave the phone alone for 10 mins, then shut down and restart again.
Hopefully all should be good but if not you have ruled out software failure and it must be a hardware issue.
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