Two things I could recommend.
Make sure you're on the latest firmware for your Portserver. You can get this firmware from http://www.digi.com/support, then enter in your product/OS combo.
If that doesn't work, make a copy of your configuration with the "cpconf term" command, then call Technical Support.
As for Realport, the driver allows you to manage the serial ports from the OS rather than via Portserver hardware settings. Perhaps that would work out better for you. Also, the driver is designed with spurious network outages in mind, it reconnects without losing any data. It may be a good thing for you to try and compare.