When you say you've, "unlocked it," I'm guessing you mean you rooted it, then I would also assume this means you have a custom recovery. If this is the case then I would advise you to acquire a copy of Titanium Backup and backup everything (and SMS Backup if you backup your texts like I do), syncing your contacts with Google and doing a factory reset. Make sure you also wipe your cache and your dalvik cache.
If the reboots persist, try a custom ROM to see if it's just a firmware issue. Odin and Heimdall flashes are also really great, but can be risky if you don't know what you're doing.
If you haven't rooted your phone then what the forget do you mean by, "unlocked it," and why would that void your warranty? Go root it and flash a custom recovery immediately if you don't give a stuff about your warranty. (Even though rooting can be undone, cause it totally can be, and I totally returned one of my phones that I had rooted previously.)
