Iphone keeps shutting off? Why?

This has happened 3 times now. My iphone 4s which I have had for 7months, I brought as a grade a refurbished iphone from a sale shop online. It came with a 12month warranty. It has suddenly crashed then the apple logo will appear and then it will go and then it will appear again. It does this repeatedly. The screen remains illuminated. I have left my phone to go completely flat before I have plugged it in to charge it and it has been okay again. I have also had several apps including my messages crashing and not allowing me to use them until i turn off the device and back on again. It has also a few time crashed but turned back on again without problems.
Why is this and how should I fix it?
Do you think I should contact the store I purchased it from to get it repaired or replaced under the warranty?

Wow thats strange, is there an Apple store there to take it to? Otherwise take it to whoever the warranty is from.

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