Re: Reboots for no apparent reason
My story) I actually had this issue with one of my old desktops. I couldn't figure it out, so what I did was remove my video card and used the system default video card temporarily. While running that I did some system checks (ram checks ETC) and removed all drivers related to the video card I installed on it. After doing that I re-installed the video card and updated the drivers for it, and I haven't had a problem since. I'm not sure exactly what I did to fix it, but my best guess would be 1 of these 2
1) Drivers where corrupted when I first installed them causing my OS to crash and fail.
2) (this one I think might be your issue) My power port on the video card was a lil bent (not saying yours is bend) causing a surge in the system, and what I've noticed with the is sometimes when your desktops have a internal power issue they tend to shut down or restart.
Solution) I recommend you try checking the connection of your new video card (power as well as direct connection to PCI slot) as well as the connection of your power supply to your MoBo. Just to see if it's a power issue... If all else fails reinstall drivers with the video card attached, or unistall the video card remove drivers and reinstall.
This is all I know from personnal experience, so if it doesn't work I appologize. But I do hope you can get this issue resolved, because I know how annoying it can be.