Is there a time that you know it turns off? Does anything related happen at that? I know this is a shot in the dark but when does it turn off?
Did you reload any latest OS or made any change that may have contributed to this issue.
Have you tried to reload the OS and see if the problem goes away?
Happiness is when what you think, what you say, and what you do are in harmony.
Blackberry Bold on AT&T : 188.8.131.521