Yosemite suddenly logs me out and in again

Question

Since updating to Yosemite I am facing some strange issues.
While surfing of Working on my MacBook Pro (Late 2011) it suddenly logs me out and immediate in again.
All open windows reappear but the whole startup procedure starts again.
All connected or opened Drives and Images need to be reopened.
As I'm working with this Computer also for my DJ-Gigs I think it's a very high risk at the moment to do a gig with this OS.
I don't want to get logged out suddenly during the audience is listening to the music.
Any guesses?
Is it a bug or a new feature which can be switched off?
I already checked for the Autologout after XX minutes of inactivity - it's not activated in my Preferences.

Answer 1

When you have the problem, note the exact time: hour, minute, second.  
These instructions must be carried out as an administrator. If you have only one user account, you are the administrator.
Launch the Console application in any of the following ways:
☞ Enter the first few letters of its name into a Spotlight search. Select it in the results (it should be at the top.)
☞ In the Finder, select Go ▹ Utilities from the menu bar, or press the key combination shift-command-U. The application is in the folder that opens.
☞ Open LaunchPad. Click Utilities, then Console in the icon grid.
The title of the Console window should be All Messages. If it isn't, select
          SYSTEM LOG QUERIES ▹ All Messages
from the log list on the left. If you don't see that list, select
          View ▹ Show Log List
from the menu bar at the top of the screen.
Each message in the log begins with the date and time when it was entered. Scroll back to the time you noted above.
Select the messages entered from then until the end of the episode, or until they start to repeat, whichever comes first.
Copy the messages to the Clipboard by pressing the key combination command-C. Paste into a reply to this message by pressing command-V.
The log contains a vast amount of information, almost all of it useless for solving any particular problem. When posting a log extract, be selective. A few dozen lines are almost always more than enough.
Please don't indiscriminately dump thousands of lines from the log into this discussion.
Please don't post screenshots of log messages—post the text.
Some private information, such as your name, may appear in the log. Anonymize before posting.