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