1. Ensure the latest firmware is applied.
2. If using port sharing:
http://knowledge.digi.com/articles/Knowledge_Base_Article/Troubleshooting-Tips-When-Sharing-a-Serial-Port
3. Check for flow control buffer overflows by issuing "display ports" from the command line of the unit:
http://knowledge.digi.com/articles/Knowledge_Base_Article/What-does-OFC-and-IFC-mean-and-how-do-I-resolve-it
4. If running RealPort, please advise what operating system/version.