In recent times, Outlook restarted three or four times a day.
I read several forums and finally solved the problem by doing the following steps:
1) Run the following program: scanpst.exe
i.e.
C:\Program Files (x86)\Microsoft Office\Office14\scanpst.exe
or
C:\Program Files (x86)\Microsoft Office\Office15\scanpst.exe
2) Click 'Start' button and wait.
3) at end if the report show errors click the 'Repair' button.