Well, if the cables are fine and the computers are fine, it's time to move further along the chain. The next stop would be the switch. Probably the easiest thing to do would be to go out and get a second 18-24 port switch and just swap it in for the current one to see what happens. Otherwise you can take whichever system is acting up, swap the connection to a different port and see if that has any effect.
I have 15 computers connected to a 24 port unmanaged switch. Whenever I try connect them all one computer is always not connecting, too make it more confusing the computer each time I flash the switch is a different one on the network, I know the cables and computers are fine as I have tested them
Anyone have any ideas???