SQL Server "network-related or instance-specific error" once a day or so (perplexed!) - linq-to-sql

Question

We are experiencing the same error as this StackOverflow Q ...
System.Data.SqlClient.SqlException (0x80131904): A network-related or instance-specific error occurred while establishing a connection to SQL Server. The server was not found or was not accessible. Verify that the instance name is correct and that SQL Server is configured to allow remote connections. (provider: TCP Provider, error: 0 - A connection attempt failed because the connected party did not properly respond after a period of time, or established connection failed because connected host has failed to respond.)
at System.Data.ProviderBase.DbConnectionPool.GetConnection(DbConnection owningObject)
at System.Data.ProviderBase.DbConnectionFactory.GetConnection(DbConnection owningConnection)
at System.Data.ProviderBase.DbConnectionClosed.OpenConnection(DbConnection outerConnection, DbConnectionFactory connectionFactory)
at System.Data.SqlClient.SqlConnection.Open()
at System.Data.Linq.SqlClient.SqlConnectionManager.UseConnection(IConnectionUser user)
at System.Data.Linq.SqlClient.SqlProvider.get_IsSqlCe()
at System.Data.Linq.SqlClient.SqlProvider.InitializeProviderMode()
at System.Data.Linq.SqlClient.SqlProvider.System.Data.Linq.Provider.IProvider.Execute(Expression query)
... except that in the referenced StackOverflow Q, they need to restart SQL Server once the error occurs - and we do not. We'll get this error once a day, or once every few days - and all is fine after the error occurs, until the next time it occurs.
This makes us think it's not a "forgot to close connections" issue. We have a moderately busy ASP.NET 4.0 WebForms / SQL Server 2008 R2 app; but we're quite positive we're not exceeding the max # of database connections.
Any thoughts on this problem, or an approach to diagnose?

Answer 1

Thought I would comment on our progress with this.
While none of the SQL Server documentation/articles/blogs mention that this error can be caused by server busyness, I found a forum posting where some seasoned IT pro named Matt Neerincx states that it can be, as follows:
Possible reasons for this error include:
1. Poor network link from client to server.
2. Server is very busy (meaning high CPU) and cannot respond to new connection attempts.
3. Server is running out of memory (so high memory usage for SQL).
4. tcp-ip layer on client is over-saturated with connection attempts so tcp-ip layer rejects the connection.
5. tcp-ip layer on server side is over-staturated with connection attempts and so tcp-ip layer is rejecting new connections.
6. With SQL 2005 SP2 and later there could be a custom login trigger that rejects your connection.
You can increase the connect timeout to potentially alleviate issues #2, #3, #4, #5. Setting a longer connect timeout means the driver will try longer to connect and may eventually succeed.
To determine the root cause of these intermittent failures is not super easy to do unfortunately. What I normally do is start by examining the server environment, is the server constantly running in high CPU for example, this points to #2. Is the server using a hugh amount of memory, this points to #3. You can run SQL Profiler to monitor logins and look for patterns of logins, perhaps every morning at 9AM there is a flurry of connections etc...
So we are presently walking down this path - reducing the # of queries that execute at the same time in some of our batch queries, optimizing some of our queries, etc.
Also, in our app connection string, we increased the connection timeout, and set Min Pool Size to 20 (thinking it's good to try to ensure some existing, unused connections for the app to grab, rather than needing to establish a new connection).
At this moment, it's been almost 48 hours without receiving the error; making us very hopeful.