Sounds like you bought a faulty device.
If it's a Hardware error that's causing the reboots, we can't help but to say try a cell repair place?
If it's SOFTWARE however, try using Odin to restore the device to factory settings, or better yet, upgrade to 2.3 using this thread: http://androidforums.com/indulge-all-things-root/468706-r915-rom-gingerbread-r915.html
Visit the All Things Root subsection, and browse through the ODIN sticky thread in there as well for more details.
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