The Modbus communication stops from time to time, and never gets back until the COM port is reconfigured. The device is still online - it's only the values that will not update. This is usually only an issue if Modbus groups are used.
If a trendlog with a very long sample time (e.g. 1 hour) is set to log a point and nothing else is bound to the Modbus device, all values from the device will be updated at the same interval as the trendlog. This is solved in newer versions.
Add a single Modbus point to a new Modbus group and then to set the group poll rate to the same as the Device poll rate. To enable polling of the Modbus point add an alarm that will not activate. This will result in polling of the device at the group poll rate.