Could you provide more information:
Does it happen at a specific time of day?
Does it happen when you perform a specific action?
Have you installed any new applications?
Have you tried running in safe mode for a day to see if the reboot still happens?
Have you tried clearing the cache partition recently?
Press and hold your phone’s power button for a few seconds until Android prompts you to turn off your phone—just as you would normally do to power it down. Next, tap and hold Power off for a few seconds until your phone asks you to confirm that you want to enter safe mode. Tap OK, and your phone will restart into safe mode.
This mode disables all downloaded software so if it continues to happen in this mode it would lend itself toward a hardware problem if it doesn't it would most likely be some software that you have installed causing the issue.