LAN Network Cable unplugged

I have a Wanadoo Livebox which connects my two computers to the internet. The
main one is connected via a Wireless Adaptor and the other connects to the
Livebox with an Erthernet cable. The computer which is connected via the
Wireless Adaptor never disconnects but the computer connected with the
Erthernet cable disconnects randomly several times a day displaying the
balloon sign in the bottom right hand corner saying that a LAN Network cable
is unplugged. The main computer still remains connected. To get reconnected
I have to switch the livebox off and on again. The problem never occured when
we first set up the computers but is progressively getting worse. I also
think that it has got worse since Wanadoo upgraded the Broadband connection -
could it be anything to do with the BT lines?