How do I prevent "Sign In to iCloud" from popping up on my screen?

I

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The "Sign In to iCloud" pops up every 5 seconds or so. No functions will work until "Cancel" is clicked, removing the box. It has been occasional since I converted from Alltel to AT&T last January (2015), but just in the last few days has it been constant, rendering my iphone almost useless.
 

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