Cross Platform Network Problem

This is not a specific Win 7 problem but it's involved so here goes!

I have four machines all connected to a switch, two wirelessly and two

wired. The router is wired also to the switch.

The two wired machines, one a laptop and one a desktop, both running Windows

XP, one wireless laptop is running Windows 7RC and the other Netbook,

Ubuntu.

Occasionally, like this afternoon, one of the wired machines and BOTH the

wireless machines will suddenly drop internet access, although access to

shared drives on the LAN is still there, yet the one XP wired laptop machine

does NOT drop internet access. The only way of returning access to the three

machines that have dropped it is re-boot the router, causing hassle to the

one machine that is still on the internet.

Can anyone tell me any reasons why this hugely irritating and totally

illogical scenario might come about?