This issue, symptomatic of the mouse cursor "jumping around the screen", is likely due to Windows detecting the serial device attached to the Digi device as a serial mouse or a HID (Human Interface Device), specifically because Plug&Play is enabled
This is a known Microsoft issue. Windows tries to detect what type of device is attached to any serial port both at boot up and periodically thereafter. The problem is that Windows incorrectly identifies the attached device and sometimes installs an incorrect driver for (such as a mouse driver). To correct this problem, be sure your Digi drivers are current, first off, so you will have the ability to prevent this problem.
Each Windows ASYNC ("serial card") adapter driver has an checkbox option for enabling a "Skip Enumeration" option which will prevent Windows from querying the Digi ports. Enabling this option is NOT recommended for any port which has a serial mouse, modem or printer attached. This is a "per port" option, so enable it on each as needed.
The Windows RealPort driver has a similar option called "Do Not Scan for Modems on Boot", which performs the same function for our Ethernet-attached serial port devices. Enabling this option is NOT recommended for any port which has a serial mouse, modem or printer attached. This is a "per port" option, so enable it on each as needed.
In both instances, the option is enabled via Windows Device Manager - expand the Multi-port serial adapters category, double-click the Digi device in question go to Advanced, Properties, select the serial port in question, and locate the checkbox. The exact location may vary slightly depending on the product. Please remember that this setting is used "per port" as needed.
Note: There is more information on the Microsoft's website. Reference Microsoft Knowledge Base Article 283063
Beginning with the v5.70 Edgeport driver (current, as of the time of this writing), Plug&Play is Disabled by default, so this issue should not happen. If running an older Edgeport driver, simply updating the driver should resolve this issue.