Could be a low file free memory issue which is causing the phone to restart randomly. Have a look at the link and try and free up some space. See if this helps with the problem. How To Increase Your BlackBerry's File Free Memory
My 8120 has been randomly restarting itself several times a day since I upgraded my OS to 4.5.0.162. I've done everything that's been advised here and nothing works. I do notice that it's usually when I am sending a lot of texts, but I've no clue how to fix it. So I am just having to deal with it.