I don't have a solution to your problem, but I can tell you what I'd do to troubleshoot...
If this is an intermittent problem then I'd suspect the drive may be on its last legs. To be on the safe side I'd start by making a backup of all your important files as a matter of some urgency.
After that I'd download the drive manufacturer's diagnostic tools and run those. This should run a far more in depth test than chkdsk (which only checks the drive's file system) and also look at the drive's SMART data.
I'd also take a look in Windows' event logs (accessible via Control Panel) for messages around the time the system has rebooted itself.
Worth bearing in mind that drive diagnostics are going to work the drive quite hard. If it is in the process of failing then diagnostics could be enough to kill it off for good - backup first!
Feel free to post back diagnostic results and anything of interest from the Windows event viewer if I can do anymore to help.
Ed.