econnect homeplugs problem

I have a pair of these which connect my desktop and router together, therefore providing me with network unctionality ie. Surfing etc.

Had them for 6mths + now and have had no problems at all. Bit of a strange one now, whenever someone elses uses the router for surfing the net etc, whether it's hardwired (xbox 360) or WiFi (brother's / Dads laptop), then my network connectivity grinds to a halt. The network connection remains active however to access to the internet, as the pages eventually time out. It has to be the plugs as i take them out and use a LAN cable and everything works ok, then swap back over to the plugs and everything is messed up again.

Any ideas?