I currently have Windows Server 2012 R2 running Exchange 2013 SP1. Periodically, both the CAS and MB server restart. These restarts seems to occur at nearly the same time each day. The resulting error is event id 41 kernel power. I am also seeing some Time-Service warnings. I found the following article the points to something similar:
http://itsalwaysmyproblem.com/2013/08/27/exchange-2013-and-bugcheck-0x000000ef/
The proposed fix is a hotfix install, but it is for Windows Server 2012. I am running 2012 R2. I will be contacting Microsoft support on this issue as well. I wanted to see if anyone else was experiencing this and to post my results.