About every 4th or 5th time I Shutdown my Windows 8.1 PC a fairly large error message appears on the screen. The key words are "Restructuring Cache Metadata". Then instead of shutting down, the lock screen reappears and the PC starts up again. Doing another Shutdown at that point almost always does a successful shutdown. Once it required still a third Shutdown. Any idea what is happening or how I can avoid this error?