are they hubs or switches??
if they are hubs, definitely replace them with switches.
I have 2 options to consider…
First, you could go to a single 12 or 16 port switch, connect everything to it. the router would be connected to it as well, but none of your computers would be.
the second option (which might be the best for you) would be to replace the hubs with an 8 port switch, connect all of the POS terminals to it. Connect it to the router, and connect your internet jukebox and internet game system to the router, not the switch. In this case, all of the POS traffic would be on their own switch with nothing else to interfere. If you are in fact currently using hubs and not switches, my guess is that all of the network traffic that the jukebox and game system are causing is creating a boatload of network collisions within your hubs, thus slowing everything down and possibly dropping connections.
As for PC 2, do you have any shared printers attached to that computer? if so, there is a remote possibility that the internet radio stuff on there might be slowing that machine down when it has print jobs to deal with.