iMac 20inch early 2008 losing internet connection on ethernet

I have an early 2008 iMac running OS X Lion (10.7.5) and more recently I face the following problem: it loses internet connection for 2-3 seconds, when I use it on ethernet, and then it connects again; the problem appears randomly, but almost everytime I use this computer in the last week. I have been using this machine on ethernet for years without any problem and I wonder if the upgrade I have made 3 months ago from Snow Leopard to Lion could have played any role to this.
I am pretty sure this is not a problem with my router, because I have tested other machines on the same port and they work flawlessly. I have also changed the cable and tried giving manually an IP address to this certain computer to no avail.
Wi-Fi works fine, but I need to use it on ethernet for stability reasons.
Any possible solutiion?