I have encounter this problem last week and i spent my day to read the xda forum. End result is to download an apps called "dummy file generator" and run the apps then select start to write dummy file on your internal memory.

During the process your phone might freeze but let it be. Your phone will freeze and unfreeze itself, but when the process is completed just select delete all dummy file. from that second onward your phone should be free from freeze problem.

I hope this can help other SGS3 user too 

i was having this problem too after i updated FB and Whatsapp. from once in a while restarts to almost every 3-5mins. it will freeze more frequent if im on the internet wifi/mobile data.

currently trying this fix.
should i do this to both internal memory and ext sd?

phone is freezing now, will update the results later.

thx modskywalker

Why bother to fix? Why not send to SC?

If your phone is still behave like before i suggest you to follow the steps below :-
1) make sure your rom is the latest rom
2) backup all your things from int and ext memory
3) connect your phone to pc and on the my computer right click to format your ext and int memory
4) download the dummy file generator and run it again (for int memory)

observe your phone 
if you bring it to SC they will reflash the rom for you and see whether your phone will still freeze. If the freeze still exist they will only replace mobo for you.

Freeze occur because of the emmc malfunction  thats why when your phone is freeze just let it be it will unfreeze and your phone will recognised it as bad block. so next time if your phone writing data to your int memory card it will skip the bad memory block and your phone will not freeze. (sorry for my bad english, I know my translation is bad but im just trying to help)

If your phone is still behave like before i suggest you to follow the steps below :-
1) make sure your rom is the latest rom
2) backup all your things from int and ext memory
3) connect your phone to pc and on the my computer right click to format your ext and int memory
4) download the dummy file generator and run it again (for int memory)

observe your phone 
if you bring it to SC they will reflash the rom for you and see whether your phone will still freeze. If the freeze still exist they will only replace mobo for you.

Freeze occur because of the emmc malfunction  thats why when your phone is freeze just let it be it will unfreeze and your phone will recognised it as bad block. so next time if your phone writing data to your int memory card it will skip the bad memory block and your phone will not freeze. (sorry for my bad english, I know my translation is bad but im just trying to help)