Two symptoms, potentially linked:

• Specific server in a farm of identical machines generates errors, apparently one per second It is logged Via elmah as:
  System.TimeoutException: Timeout waiting for a MongoConnection.
  at MongoDB.Driver.Internal.MongoConnectionPool.AcquireConnection(MongoDatabase database)
  at MongoDB.Driver.MongoServerInstance.AcquireConnection(MongoDatabase database)
  at MongoDB.Driver.MongoServer.AcquireConnection(MongoDatabase database, Boolean slaveOk)
  at MongoDB.Driver.MongoCursorEnumerator`1.AcquireConnection()
  at MongoDB.Driver.MongoCursorEnumerator`1.GetFirst()
  at MongoDB.Driver.MongoCursorEnumerator`1.MoveNext()
  at System.Linq.Enumerable.FirstOrDefault[TSource](IEnumerable`1 source)
  at MongoDB.Driver.MongoCollection.FindOneAs[TDocument](IMongoQuery query)
  at MongoDB.Driver.MongoCollection`1.FindOne(IMongoQuery query)
  at MongoDB.Driver.MongoDatabase.RunCommandAs(Type commandResultType, IMongoCommand command)
  at MongoDB.Driver.MongoDatabase.RunCommandAs[TCommandResult](IMongoCommand command)
  at MongoDB.Driver.MongoDatabase.RunCommand(IMongoCommand command)
  at MongoDB.Driver.MongoCollection.Count(IMongoQuery query)

This is resolved by resetting the app-pool for that site on that host. No specific trigger is found on the servers themselves in either the mongodb.log files nor from other system monitoring tools.

• Some time later (30 minutes)all servers in the pool (of 6) begin logging:
  System.IO.IOException: Unable to read data from the transport connection: 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. ---> System.Net.Sockets.SocketException: 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.Net.Sockets.NetworkStream.Read(Byte[] buffer, Int32 offset, Int32 size)
  • • End of inner exception stack trace —
      at System.Net.Sockets.NetworkStream.Read(Byte[] buffer, Int32 offset, Int32 size)
      at MongoDB.Bson.IO.BsonBuffer.LoadFrom(Stream stream, Int32 count)
      at MongoDB.Bson.IO.BsonBuffer.LoadFrom(Stream stream)
      at MongoDB.Driver.Internal.MongoConnection.ReceiveMessage[TDocument](BsonBinaryReaderSettings readerSettings, IBsonSerializationOptions serializationOptions)
      at MongoDB.Driver.MongoCursorEnumerator`1.GetReply(MongoConnection connection, MongoRequestMessage message)
      at MongoDB.Driver.MongoCursorEnumerator`1.GetFirst()
      at MongoDB.Driver.MongoCursorEnumerator`1.MoveNext()
      at System.Linq.Enumerable.FirstOrDefault[TSource](IEnumerable`1 source)
      at MongoDB.Driver.MongoCollection.FindOneAs[TDocument](IMongoQuery query)
      at MongoDB.Driver.MongoCollection`1.FindOne(IMongoQuery query)
      at MongoDB.Driver.MongoDatabase.RunCommandAs(Type commandResultType, IMongoCommand command)
      at MongoDB.Driver.MongoDatabase.RunCommandAs[TCommandResult](IMongoCommand command)
      at MongoDB.Driver.MongoDatabase.RunCommand(IMongoCommand command)
      at MongoDB.Driver.MongoCollection.Count(IMongoQuery query)

This is again resolved by an app-pool reset.

In both cases the evidence points more to the driver, however we would like to know how to go about directing ourselves in one direction or another. A problem local to one server doesn't feel like a server issues, however a problem shared among several could be.