Imac loses internet connection within 30 seconds

[macrotto]

I have had an IMAC for 1 year, connected to a home network next to a PC, connecting to the internet flawlessly. this past week my internet connection has begun to fail. It is connected as i can get to different websites but the loading of websites slows within 30 seconds. when i download a file the speed starts at 50 kbs/sec and slows to 0. I can reboot my machine, can get a website loaded again but if I download and/or change to another website it is very slow. I tried www.speedtest.com and it couldn't load the test results because it took longer than 30 seconds. I have tested the ethernet line and it works fine with my laptop and pc desktop. Ironically I'm using my pc to write about my mac on a macblog. help?