Ubuntu disconnects internet

[catabriga]

I have recently installed Ubuntu (hardy amd64) and I'm having issues with the internet. The problem happens with both cable and wireless. After a few seconds after I start a download my internet disconnects. After that it reconnects and restarts the download, then after seconds it disconnects again.

The strangest of all is that I had to reinstall Ubuntu, the first time I installed it there was no problem at all, then when I reinstalled this problem appeared. I even tried reinstalling a third time but the problem was not fixed. There is no problem when using internet in windows or even in an older ubuntu in the laptop. So... what can I do to fix that?

[catabriga]

I have done some more tests and found that this problem doesn't occur if I connect directly with the modem, no router. So I guess the problem is with the router.

Now, does anyone know what it might be? My internet is disconnect every time I try to download something, there's no problem browsing web pages and there's no problem also doing anything in the internet using windows. The problem happens with both wired and wireless connection. Please help, it's really important to solve that.