my macbook air turns off continuosly

13.3/1.7/8/512FLASH-ITA

MacBook Air 13,3" with Intel Core i7 dual-core a 1,7GHz 500 Gb 13.3/1.7/8/512FLASH-ITA

OS X 10.10.3

Installed applications:

MS Word

Skype

my macbook air in waranty tuns off by itself 5 times a day.

I bought it on May 2014 and after while months it started to turn off. I contacted the support service and I followed all the instructions that they gave me to solve out the problem, included the re-installation of the OS. However, the shutdowns started again to be frequent. I sent the laptop to the assistance service and they gave it me back telling me that the file "vault" was not codifying. After a day the laptop started to turn off again.

How can I obtain to have a working macbook air?

Thank you,

raimondo manno