I have a user whose Blackberry 8310 keeps turning off of it's own volition. He states that it usually happens when he's on a call - and right before the phone looses power, he states that he and the person on the other end of the line can hear a loud buzzing. At this point, the only way to turn the phone back on is to plug in a charger.
Of course, our first call was to our provider, AT&T. We done a warranty replacement (refurb unit); not once, not twice, but thrice. My user has also replaced the battery with a new as well as the SIM card.
The only new thing about the account is a recently added overseas calling plan, which however unlikely, I've removed to rule out anything.