sniffer pro is no different than ethereal, or any other packet capture program..
Span the port the unix box is on to another, setup the sniffer PC on that port, and then pull a trace...
If you believe the issue to be further downstream of the switch the unix box is on, you would need to move the sniffer to a more advantageous position..
If you want I could post or email the method used to filter out just the traffic coming out of the unix box, or filter based on two devices..
packet-level.com has alot of sniffer stuff, I have other stuff I could make available too on how to use it..
what exactly is the problem, and what are you hoping to be able to capture and see ?