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I have this on my 4s. It's hardware, not software (already tried fresh install few times). Finally I used tweak (jailbroken) to override the switch because it already out of warranty. Sorry, not really exactly same with your case.
 
The strange issue is, only realised when I installed iOS 9.2. Before that no issues.
 
I'm not experiencing this on iOS 9.2 on my iPhone 6 Plus.

Perhaps try a restore.

If it continues it could be hardware related then check in with a Genius Bar at your most local  Store.
 
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