Ubuntu Networking :: Internet Drops Out For Firefox

We have three computers at home. I have the only one running Linux, the other two are running Windows XP and using Belkin Wireless G USB Adapters. I started using Linux around November, and used a powerline ethernet cable. (It consists of two parts. One part you plug into your router and the power socket beside it, and the other you plug into your computer and the power socket beside that. It then uses the power lines as a network, and acts like a normal ethernet cable. I used the 'Auto eth0' connection without much problem). Back then, my internet would stop working once every few hours but only for firefox. Other programs such as bittorrent and Spotify continued working.

However, since I got back from holiday a few days ago the situation's worse. The internet regularly pauses (every few minutes, including spotify and msn) for about 30 seconds, and then comes back. Downloads are not cancelled, as they are when I am disconnecting from the internet, they merely seem to pause. The other two computers in our house don't have this problem, and I have tried the wireless adapter, yet it seems to pause at the same times as the wired connection. I know that it too used to work back in November.