My Samsung galaxy S3 mini was getting to "freeze" after the latest update - 4.1.2
It would also restart whenever it feel like it.
When the batty is almost new, I was trying to put an newer system version on it all by myself with the programme "Odin". What a stupid thing to do. I couldn't make it work, so I figured out that a hard root might do it - But there is no version to do the root from when I follow the instructions. Now I only can make the phone react if I put it to download mode.