increase the thread pools a bit, that will help you I think.
Our system is running over one year without any problems (restarts only for updates). 268 Data Sources, 32,359 Data Points and 9 publishers.
Another thing I do for each mango-installation is to config the Mango-internal-data-source to the max (Mango process CPU load, System CPU load etc.) to get an overview what happens with the system and the data-sources (= PLCs). Don't forget to set "override system purge settings" for this ds or you will get a very large database...
Also check your update period versus poll duration for the modbus-ds. Maybe you have to increase your update period because the PLC needs to much time or the network-traffic is limited?