Sounds like it's a non-Sonar problem and could be caused by any number of things. Start checking the obvious - download Memtest & check you RAM
+2
The problem is most likely RAM related... could also be power-supply related (if it's too lean for the machine)
Uncheck "Automatically Restart" and the system will blue screen.
Don't be alarmed when it happens. It might list the exact culprit (look at the error message).
Control Panel>System>Advanced System Settings>Startup and Recovery (click "Settings" button):
Under System Failure... uncheck (disable) Automatically Restart
As was mentioned, test the RAM.
I'd use Memtest... and test a second time with an overall stress-test (using something like Prime95).
If you get an error with Memtest or Prime95, you need to rule out the bad stick/s.
To do this, test a single stick of RAM in the outer most RAM slot (the one closest to the outer edge of the motherboard - furthest away from the CPU). Repeat this test for each stick of RAM. When you encounter a bad stick, the machine will likely not boot.