This shows that your system is experiencing memory pressure at that time. Have you confirmed that nothing new was installed on your box that could be causing external memory pressure on the SQL Server? Have you started any new processes that could be running at the same time? Has an old process started taking longer and is now running at the same time. Do you have enough memory for your system? You can following the troubleshooting steps in this article to see if you are having memory issues (search for your error message):