Inspiron 5100 + Targus PA090 Port Replicator, Serial Port not recognized in *some* programs

I'm a happy owner of a new 5100, and finally installed the PA090 port replicator tonight to set up my external monitor, keyboard, and serial port.

It all is working well, except that certain programs don't work with the COM4 supplied by the replicator, while others do just fine.

For example, HyperTerminal is able to open the port and access a GPS hooked the serial port.

Garmin MapSource City Navigator tries, but comes back with a "Parameter Error."

EasyGPS returns the exact same error.

Using the PortMon utility at:

http://www.sysinternals.com/ntw2k/freeware/portmon.shtml

I am able to capture the serial port commands and traffic to COM4. The application is able to open the serial port and initialize its parameters up to a certain point. However, one command being sent results in a error return from Windows XP:

26 0.00000000 MapSource.exe IOCTL_SERIAL_SET_QUEUE_SIZE U2SPORT1 InSize: 4096 OutSize: 4096
26 0.00000140 INVALID PARAMETER

This suggests that the serial port driver for the PA090 either doesn't implement this command or does but is broken. I've updated the driver to 2.24 from the Targus website, but it is still having the same behavior.

When a (different) application is successfully using the serial port, the captured traffic does not contain this command.

Any developers out there able to recognize these symptoms?

-Johnathan