sometimes the "shutting off" problem is caused from a simple thing like dirty battery connections.

remove your battery from the phone and make sure the contact points are clean. you can use a pencil eraser to get rid of residue or use rubbing alcohol on a q-tip.

if that doesn't work then you are dealing with a phone issue for sure - in which case your best option (unfortunately) is to bring it to a service center for what is sure to be a costly repair.

good luck.

i have the same problem with my phone, which is why im buying a new one, cause i think i dropped it one too many times, but it does shut off alll the time for no apparant reason, i thought it was just mine but i guess not. lol