had this problem too. my phone would shut down after about 10-20 minutes of use (web videos, surfing) then reboot anywhere from 1 - 8 times in a row until i did a battery pull for about 5 minutes and even then sometimes it would reboot. I finally went to verizon store and they said we will try the battery and if that's not it then we will replace the phone, well i got the new battery, put it in and 10 minutes later it rebooted, so its not the battery, I went to the store again today and they are giving me a new phone. I also thought the back bottom of the phone was getting warmer than it did when i first got the phone.
Looking forward to getting a new working phone.