Samsung Galaxy S3 Shutting down when screen idle

Hi
My Samsung Galaxy S3 keeps shutting down when screen goes idle. I've done the new battery, deleting apps and hard reset.
Phone continues to stop once screen goes idle. Runs fine the rest of the time, even for an hour straight when driving around with the navigation software running, two minutes with the screen idle and no turn on. Takes a long time to start if if it goes off.
GT-I9300
Op sys- 4.3
baseband:I9300XXUGNA8
kernal: 3.0.31-2372379 dpi@HP19#1
build number: JSS15J.I9300XXUGNB4
Optus Australia phone- unlocked and on Vodaphone Australia.
Any help would be greatly appreciated.
Thank you.
Derek

Update....
After the above hard reset, I restored most of my stuff from backups, not all my apps, just the ones I used most. Problem started again.

Did another hard rest (including wiping the cache) and rebooted without the sim. This lasted with no problem for 12 hours, working fine (nothing restored to the phone) so I shut down and added the sim. Reboot and working fine for over 12 hours. Restored contacts and 12 hours later is still going fine.

Hi Sinkster
Thanks for the reply. The phone is as up to date as the software allows and has reverted to shutting off with nothing but contacts put back on it.
Got a new phone form my service provider today. Not sure what to do with the S3 now. Might try cyanogen and have a play around unless something else comes up that works.