5 different computers shows Netword cable unplug

It started last friday with a call from a client that one of the computers on the network cannot connect because the system thinks that it is not connected to the network cable unplugged. not a problem I went and checked the system and it was not so the cable was plugged into the switch along with 2 other systems they were fine I checked the system hard ware it checked out but to save time I installed a PCI ethernet card and plugged in the cable system came up and all was fine. On Tuesday I got another call from the same client with another computer with the same problem I allpied the same fix and all was good today I got another call from the same client 2 other computers are doing the same thing this time different brand computer does anyone know of a worm that will disable the network connection and give the error cable unplug preventing network access. and what I could to correct it?