I encountered a similar issue a few weeks ago, my phone kept rebooting now and then. Knowing about the "sudden death" issue, I verified and it all was OK. My problem was a hardware one. The power button had an issue, so almost whenever I tucked it into my pocket it rebooted. Continuously. Even after I removed the battery and put it back it kept rebooting. I took it to a GSM service and I had it fixed in 10 minutes for about $10. They replaced the power button (the plastic and that rubber-like thingy). I'm glad they didn't have to remove the mechanical button. Also you could check the battery contacts (on the battery itself) and clean them - use something hard that can wipe off any residue.
Last edited by alex.sg; 27th January 2014 at 11:33 PM.