Internet issues

Help. I am running on snow leopard 10.6.3 and my internet is not quite stable. My mb is Asus P5N73-CM.

I am using a wired internet connection. The internet disconnects whenever I go to certain websites or run certain programs such as yahoo, hotmail or logon to msn messenger or bittorrent.

When I lose connection, I open up Network Diagnostics and I see Ethernet and Network Settings green, ISP, Internet and Server are red. I cannot even access the router through internet browser. (I tried both connecting to the router and directly to cable modem, results are the same.)

I googled a bit and found that a workaround is to go to Network, turn off Configure IPv4 and apply and then select Using DHCP and apply again. The connections come back on within seconds. However I couldn't find any solution that fix the problem permanently. I also tried deleting library/preferences/systemconfiguration folder and also created a new
Location in network, I still get disconnected.

I have no internet problems when I use a mobile broadband dongles, thus I suspect is a ethernet problem.

Has anyone encountered or heard of the issue? Is there a permanant solution to my problem? Will buying a new ethernet card and installing it work?