My iphone 4 keeps randomly turning off?

This started 2 days ago at the same exact time. I haven't dropped it, nor shaken it in any violent manners. It will turn off when i'm using it, or when i take it out of my pocket, i notice its off. When i hold the powerbutton, it won't turn on. When i hold the power volume plus home button, still nothing. Sometimes itd turn on and just show the apple symbol, then turn back off. If i hit it, itd turn on, and stay on for the rest of the day. But then the next morning this happens AGAIN. Its annoying. Can i fix this? Myself, or genius bar or a kiosk etc? If i take it to apple will they charge me just to have them check what's up?

That has happened to me before (I have an iPhone 4).
First you have to turn on your data, then take out the sim card and put it back in and then turn off your data and it should be fixed!

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