SE Z200 problem

[jbt]

Hi, i have a SE z200 that is playing up. when the phone is in silent mode it works perfectly (Except does not vibrate), however when in normal mode there is a serious problem. Whenever somebody tries to call the phone turns itself off. Why could this be? i believe that is is due to the firmware and by reinstalling this the problem could be solved. Does anybody have any suggestions to help me solve this problem?? Thanks in advance.