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We identified a problem in some versions of Windows Phone 8.1 that could cause a phone to restart periodically. We've already made a fix that has resolved this problem for the majority of Windows Phones. However, some phones that have not connected to Microsoft servers still need the fix to be applied.
First, check to see if you have an update available. Go to the App list, tap Settings
Download the Stop Restart app to your phone, open it, and then follow the steps in the app. (For best results, connect to Wi-Fi.)
The app will display either of two results:
Even if the app fixes your phone or says your phone is OK, you may still be offered a critical update. Find out more about the critical update
Article ID: 12626 - Last Review: 23 Aug 2016 - Revision: 11