intermittent loss of internet with wired PC

So my boss has a main server room that runs his POS system, he has a switch that all of the POS systems are connected into that connect to the main server, that is then wired directly to the modem. Then there is a router that is wired for just wireless use, and then my bosses computer is wired directly into the modem, his internet has been dropping often and he really didnt do anything about it, but they recently got a new modem and the internet on his computer has been dropping more often that it has become a big inconvenience for him, a few weeks ago he had the lines redone from the server room to his computer and is all new cabling now, and had all of the jacks on the walls checked, i thought it could possibly be a wiring problem, but it is all working. His computer is the only thing that has internet problems, the internet for the POS systems never have a problem, but his computer does. He also says that sometimes when he checks the connection it will say network 4 but other times it will say network 2. I have no idea what to do, and it would really put us on good terms if i could help fix his problem.