acceleport 16em send/transmit inconsistency

I have a windows 2000 Pro O/S with an AccelePort 16em attached to communicate with multiple devices at 9600 baud. The messages from the serial devices are supposed to be spaced every 10 sec. There is bi-directional communication. I have the default portserver configuration settings in device manager. Usually, when I receive a message from the serial device, I respond in 3 sec. By analyzer, it is obvious sometimes I respond in 3 and sometimes 20 seconds. Also, sometimes two messages come from the digiboard that were spaced from the serial devices 10 sec apart. What config settings do you suggest for this type of application?