need help for s4

A

AC Question

hey guys,i bought this galaxy s4 from ebay in Italia about 8 months ago. But from few months i am having this problem that it switches off itself if i use my phone for more than 5 minutes ( if I use camera and just clicked 2/3 pics , if I use MP3 player and try and skip some songs).
I tried to switch it on but it won't unless I plug it to the charger.I don't know what's wrong with it.I even tried to ask the place where i bought it and they said to go to Samsung centre and when I went to Samsung centre, they said this phone was made in France so they can help me with it. Can anybody tell me what's wrong with it.Its really annoying.

P.S. recently i noticed that the battery was little bit fatter..So will it help if i only change the battery.or may be its a damaged piece?