Moderate internet use causes failure to connect on one computer only

I dont really understand why this would happen, but it is causing some grief in my house, so any help would be appreciated.

The setup:

24meg internet (runs at 1mb/s due to location) Ping is usually very good.

5 computers connected.
1 pc (dad) ethernet cable (the computer that has problems)
1 mac (usually turned off)
1 pc (brother) 20m long ethernet cable
2 pcs (mine) 20m crossover cable into a network switch.

The problem is that whenever me or my brother use the internet (streaming video from iplayer for example) My fathers computer simply refuses to connect to any website. Even google or other very small sites. This problem doesnt happen on any other of the computers.

His solution is simply to pull everyones' plug out, which you can understand, is very annoying. It has got to the point where any slight delay on his computer results in all the plugs coming out. I have tried to explain that we arent using all the bandwidth, but he sees that this fixes his problem, which it does, so obviously I am intested in finding a solution.

Any idea why this refusal to connect would happen on just his computer?

I have tested by running a download capped at 100k/s (10% of the download speed) and this problem still happens, although not always.

 

The router's firewall is off in these cases.

I dont think upload is the problem, this is generally happening while there is no uploads in progress.

Yes he is connected but nothing loads. basically the pages time out. At precisely the same time my computers can load webpages instantly. As soon as he is the only one connected the pages load fine.