The MaxClientConnections in the parameters section controls the number of SQL server threads the Virtuoso server is allowed to use for external and internal requests. If your application requires more server threads than the Virtuoso server configuration allows then. you will start hitting bottle necks which would explain the intermittent behaviour being experience. As indicated in the Virtuoso "MaxClientConnections" configuration parameter post, it is recommended this be set to at least 100 typically, although it can be set higher as the threads are allocated on demand ie as and when required.