How to fix an iPhone that keeps restarting - Panic Log and iDeviceLogAnalyzer 

You may find that your iPhone boots up fully but is restarting periodically over a set period of time, most commonly after 3 minutes. The first step to solving this problem is to examine the panic log within the iPhone's settings. You can find this by navigating to Settings > Privacy > Analytics & Improvements > Analytics Data > panic-full-(date).ips

After pressing "Analytics Data" remember to scroll down until you find the "panic-full" field. It can be far down the list sometimes.

Your Panic Log

"Userspace watchdog timeout" is one of the most common hints at this problem. What does this mean? 

We can find the answer by downloading the iDeviceLogAnalyzer tool. 

Link -iDeviceLogAnalyzer tool 

This tool will interpret all the text and tell you the likely cause of your iPhone restarting. To download the tool, simply click the iDevice Panic Log Analyzer field under the "Releases" section as indicated below.

iDeviceLogAnalyzer