In the server logs I'm seeing this error:
SQL Server is terminating because of fatal exception c0000006. This error may be caused by an unhandled Win32 or C++ exception or by an access violation encountered during exception handling. Check the SQL error log for any related stack dumps or messages. This exception forces SQL Server to shutdown. To recover from this error restart the server (unless SQLAgent is configured to auto restart).
Restarting the server recovers it, but it has happened a couple of times in a week and each time it stopped for many hours before it restarted automatically (I've had to set up the auto restart in the services because SQL Server Agent is not present in the Express edition)