Try to start with this:
1. Properties of My computer label on desktop
2. Advanced tab
3. Startup and Recovery settings
4 Checkbox "Automatically restart" must be turned off.
If that checkbox is on turn it off. Then, instead of rebooting, computer should show you some BSOD. If you lucky you will can detect what is the cause of problem.
And you can always try to install only Antivirus component of SEP on troubled PC, if you dont already did that.