i had the same problem since last week.. phone keep restarting on its own. sometimes it just loop at the samsung logo until i hard reboot it. 2 days back, i tried to reflash the ROM using stock (always on stock) but in the process i think my /efs folder get corrupted.
now my IMEI is messed up and can't get the phone to register to network.

tried many methods but i dont have the /efs folder backed up before, just nandroid backup.

found a way to edit the hex on the nv_data.bin but the tutorial is not for S3.

anyone kind enough to upload the nv_data.bin file in /efs folder for me to make reference? tutorial show only 3 lines to edit, and i want to try this as the last resort.

mine is stock as well. just rooted.
found a thread in xda with the same reboot issue, also within this few weeks.

timebomb in s3?

thinking of getting N5 if i can't revive this S3. sad

if still under warranty then send in.

i dont have warranty anymore. lol

now samsung 4.1.2 I9300XXEMF6
from samsung 4.1.2 I9300XXEMC2

using odin, the usual step. not the first time using odin. never had /efs backup before.