Here is a possibility, since I've had this happen years ago on my original Galaxy S.
When does your phone reboot itself? Is there a particular place in your city? Does it only do it at home? Does it only do it at work? Can you determine how often?
It is entirely possible that there is a problem with your carrier's tower that you're connected to. If the tower is sending bad packets, this could cause your phone to reboot at random. I, and several of my co-workers each had the same phone and when we were all at work, our phones would reboot at least 4 times each day. After much troubleshooting, reading and research, we decided that there must be a problem with the tower near our office.
Each of us, one day after the other, called in and spoke with technical support. NOT the front line customer service, the actual technical department. We all reported the same thing - "my phone keeps rebooting for no reason at all, and so do my co-workers". After a few conversations, and gently nudging their tech support, they finally agreed that something may be wrong at the tower so they dispatched a repair crew and after about a month of calls, it was fixed and they DID call me back and tell me that they did correct a problem.
I have no clue if this is what your problem is, but if you can't find any sane reason, call your carrier's tech support and ask them. ESPECIALLY if it's only happening when you're in one specific location.