Is crash dump enabled? I'm wondering if it's making a dump file. If you have a dump file we can analyse it for you and possibly find the problem.
1) Click Start, then Run and type in sysdm.cpl and hit enter
2) Click on the Advanced tab and then under Startup and Recovery click the Settings button
3) Verify that a Small Memory Dump is selected
If the system is catching the error then you should see files in your Windows\Minidump folder that correspond to the dates/times that it has crashed. We can analyze these files for you and possibly figure out what device driver or hardware issue is causing the reboots.