I faced issue with Windows Server 2008 and Server 2012 Task Scheduler stops running scheduled tasks.
In windows Server 2008 the Task execution never starts and in Server 2012 Tasks execution stopped after some time.
After a Long hair pulling and head banging with Microsoft Forum, I eventually (infect accidently) found the solution.
There is a Checkbox while in properties of Task “Run with highest Privileges”,
Please check that and you are done.