Suddenly black off reset

I have iPhone 6 - 64 GB (used 25 GB) with ios 8.4.

Suddenly, in the middle of opening application for example Path, i scrolling timeline of Path, my iPhone suddenly dead (the screen was black, then the apple logo screen came out like the phone reset it its own). That happens too when i open whatsapp and listening a voice message.

I ever tried to reset the phone (from setting and reset all settings) but the problem come again.

please help me and thank you.