Suddenly turn off Iphone 4

Hi guys my IPhone 4 suddenly turned off.This happend many times.sometime phone ture off while i m on a phone call.it is hard to turn on it again .at that time, i holding the home and sleep/wake button for few secconds sometimes it turn on but sometime it doesn't .so i updated it in to ios 6.0.1.but the issue is not fiext yet. Please help me to fix this issue.😟