Moxa serial-to-ethernet connection problem

We have a Windows 2003 Server (32-bit) running in VMWare and connecting to 2 Moxa NPort 5450 serial-to-ethernet devices. We are experiencing a sporadic "disconnect" with the serial devices attached to the Moxa and they do not reconnect in our software (the software we are running is called Uniwin) until the Uniwin service installed on the server is restarted. While waiting for a Moxa rep to return my call for support, I decided to call Perle to see if they've experienced something similar - their pre-sales engineer said he knew exactly what problem was, but since it was not a Perle product he would only tell me that it had to do with a setting or some configuration of the tcp/ip stack in Windows Server 2003. Does anyone know what he might be referring to? I've tried the tcp/ip stack reset, but it sounded like it was a specific setting (possibley in the registry) and not just a "reset". Any help is greatly appreciated - thanks...