It sounds like your phone is rebooting on its own. It shouldn't be doing this. I would suggest you try powering off the phone completely. Then you should remove the battery and check that the contacts aren't dirty. Most likely they aren't, but since you are doing a battery pull, you might as well check. Leave the battery out for at least 20 seconds. Then put the battery back in and start the phone again.
See if you are still experiencing these restarts periodically. If you are, then I would suggest you exchange it for a new one. There is something wrong if it continues to randomly reboot.