Losing network connection on iMac

[krotto]

for the past year my imac as worked flawlessly. for the past week my imac shows it is connected to the internet through the built in ethernet but it loses it speed after about 30 seconds. I test it through trying to download a file. it shows that my initial speed it 60kb/sec and then slowly goes down to 0 and says download has ended. I have tested the cables on my PC and the connection is strong and doesn't waiver (tested on both a PC desktop and laptop). I can't tell if it is a OS issue, a cache issue, a hardware (ethernet)...it acts strong initially then loses power. whether I'm downloading or going to Itunes...it loads the site and then can't download anything as the speed has been lost. Upon reboot it starts over again and eventually loses speed(about 2 mins)

[michaelsanford]

The first thing you need to do is isolate the cut point: is it your router, or is it your ISP's internet connection? Do you have a home router, or is your high-speed modem connected directly to your computer? What kind of router do you use?

Open Network Preferences and watch the lights. Does your "Ethernet" light go yellow or red when your network connection goes flakey? If it stays green, it's more likely that your internet connection is failing somehow (bad modem, whatever).