Erm, are you using a router? The way you describe it seems to be a wireless LAN affair that connect 2 computers, one of which is running broadband internet connection sharing. Or have I read it wrong and it is a wireless router which is connected to your broadband modem (or another router) and acts as a gateway for 2 computers which have a wireless connection to it? Does the upstairs PC have unaffected internet access whereas the downstairs one has intermittent internet access? You could try continually pinging the other computer (or router if it is a router) to see if your wireless connection is dropping packets (at the command prompt type: ping -t). You could then position the aerials of the wireless LAN setup so it drops the least packets. Better still, connect it all with CAT5 cable.