My iphone turns off suddenly, what i have to do?

Hi:

Is installed on my iPhone the iOS 7.0.4 version but suddenly it turns-off and re-starts automatically (as become when i turns it off and turns it on).

I am worry about because i would like discard any hardware missoperation.

More information:

I bought it in September 2013, it is in good condition (no damages, no falls, no chrash, no one service), the fail began to appear since last december.

Please, make me a suggestion what i have to do or i have to claim for warranty.

Regards

Aa Zamora