In the last couple of weeks I've had a lot of Monitoring Stopped alerts which say the monitoring stopped and started on the same second:
SQL Server authentication failed at: 11 Mar 2014 11:01:39 AM
SQL Server authentication successful at: 11 Mar 2014 11:01:39 AM
I've had 36 occurrences since the 25th of February and they are very annoying (and slightly concerning). They often (but not always) happen in pairs of around 5-20 minutes apart.
I've had no other problems or errors so I don't think there is actually anything wrong with the permissions or the account.