Iphone 3gs problem, keeps rebooting and restoring itself?

I just got an iphone 3gs from my friend and it was on ios 5.1.1 and it keeps rebooting on its own plugged in or not and when it reboots it brings me back to the set up menu when you first restore it. So my friend tells me I should update it to ios 6 maybe that would help. So i update it, and everything seems like its fine then like 20 min later it did it again. And it keeps doing it every minute or so. And when it reboots it brings me back to the set up menu. Any solutions or fixes?

He probably got water or sand in it

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