They could be signs flash component is not good ...
Check with this software and see if the component is problematic
https://play.google.com/store/apps/details?id=net.vinagre.android.emmc_check
If so, it's only a matter of time until the phone stops working and then have to replace the component.
This is called "sudden death"
We repair this problem and our site has an explanation for this failure:
[http://fixmart.co.il/device/%d7%9e%d7%a2%d7%91%d7%93%d7%aa-%d7%aa%d7%99%d7%a7%d7%95%d7%9f-%d7%92%d7%9c%d7%a7%d7%a1%d7%99-s3|Galaxy S3 repair]