72 pages of people complaining about the same problem here
http://answers.microsoft.com/en-us/...t/5256ebee-2c77-4003-87be-1597ab10cb4d?page=1
The most recent cumulative update KB3093266 was suppose to address this issue and apparently did not and it apparently is still a mystery.
Some folks have apparently linked it to third party software, especially items like Security Suites from various vendors and the fact that Safe Mode seems to be an interim fix would suggest that they are on the right track.
I would typically recommend a clean boot to help further investigate the issue but unfortunately it is so difficult to diagnose because it seems to only occur once in a while and after booting into safe mode and then rebooting normally it seems to go away for a while.
I would recommend that on the next occurrence you use the clean boot method described here
https://support.microsoft.com/en-us/kb/929135
Restarting your machine without any Startup items or non-Microsoft services running and then proceed from there by slowly, item by item returning them to their previous state rebooting after each change to test the impact if any.
It's time consuming and a bit labor intensive, but it certainly would be nice if someone with the actual problem could help zero in on the offending program or process.