I have a Samsung S3 too - although I've never experienced any bugs on the scale you are talking about. I wonder if your phone is actually faulty or perhaps
you have installed some crapware thats futzing with it.
I did install some software from Samsung App Store. I was also thinking that there could be a fault in hardware but bugs I found are reproducible. It just seem to be sensitive to certain touch screen actions and lockup and finally reboot (I assume a watchdog comes to rescue).