I would open Event Viewer as administrator and look for errors it may show. Look under Admin Events, Application, System, and Hardware Events.
If it happens after 30 minutes consistantly, I'd look at your Power Option settings and set both to Never then see if this stops. If not then I'd look under the Advanced settings especially the USB and Multimedia settings.
You could also be having heat issues. If you don't have it, download CoreTemp
and check the processor temps with it.