I would like to use a network tap and some type of packet capture utility to analyze the traffic between my customers PC and their satelite modem. Can anyone give me a quick explaination of what to do?

Here is my problem. I install satellite internet access systems and sometimes the customers connection is slow. The system will work fine while connected to my laptop, but when I connect the system to my customers PC it is noticable slower. So then the customer gets upset and does not want to pay me for the install. I am trying to find a way to show the customer what is going on.