Iphone 4s white keeps turning itself off

Title says it all...

Ever since i got it, its been working perfectly.

But for the past 5 days it keeps turning itself off when im trying to delete an image, helf way through a book, texting friends and even taking pictures.

It just turns itself off and comes up with a loading symbol and it even has 50% battery or more everytime. i never can turn it back on for at least and hour..

Ive done a reset to normal settings using my itunes but it still turns off.. any advice guyss

or should i just phone up orange (network provider) and get them to send me another one ???

Thanks in advance